[BB-2882] Add `main-color` to course info

This fixes the issue where the link color was set as the text color for all the buttons.

Testing instructions:

Clone this repository in a directory accessible inside the LMS devstack. The src folder which is at the same level as the edx-platform, devstack directories is a good candidate for this as it is mounted in the LMS devstack at /edx/src.
Check out this PR branch.

Create a common-variables.scss file in the <repo dir>/lms/static/sass directory with the following contents.

$footer-bg: #dfdcdc;                                                                                                                                                                    
$header-bg: #f56b6b;                                                                                                                                                                    
$link-color: #ff0000;                                                                                                                                                                   
$main-color: #00ff00;                                                                                                                                                                   
$footer-color: #000000;                                                                                                                                                                 
$btn-primary-bg: #f8e71c;                                                                                                                                                               
$btn-register-bg: #c34d4d;                                                                                                                                                              
$btn-secondary-bg: #ff0f0f;                                                                                                                                                             
$btn-primary-color: #b42d2d;                                                                                                                                                            
$footer-link-color: #d73434;                                                                                                                                                            
$btn-register-color: #b82222;                                                                                                                                                           
$btn-secondary-color: #3a99c5;                                                                                                                                                          
$main-nav-link-color: #000000;                                                                                                                                                          
$user-dropdown-color: #bb5c5c;                                                                                                                                                          
$btn-primary-hover-bg: #d41d1d;                                                                                                                                                         
$btn-register-hover-bg: #b62323;                                                                                                                                                        
$btn-secondary-hover-bg: #349ecf;                                                                                                                                                       
$customize-register-btn: True;                                                                                                                                                          
$btn-primary-hover-color: #4ea8d1;                                                                                                                                                      
$btn-primary-border-color: #bd10e0;                                                                                                                                                     
$btn-register-hover-color: #c52c2c;                                                                                                                                                     
$btn-register-border-color: #bd4949;                                                                                                                                                    
$btn-secondary-hover-color: #d15151;                                                                                                                                                    
$btn-secondary-border-color: #4fa0c5;                                                                                                                                                   
$home-page-hero-title-color: #313131;                                                                                                                                                   
$home-page-hero-subtitle-color: #646464;                                                                                                                                                
$btn-primary-hover-border-color: #5bbdeb;                                                                                                                                               
$btn-register-hover-border-color: #c54444;                                                                                                                                              
$btn-secondary-hover-border-color: #d14a4a;                                                                                                                                             
$main-nav-item-border-bottom-color: #1a1ae6;                                                                                                                                            
$main-nav-item-hover-border-bottom-color: #ad4040;

Create _lms-overrides.scss in the same directory with the following content
```
@import 'common-variables';
```
Create partials/lms/theme/_variables-v1.scss (relative to the same directory) with the following content.
```
@import '../common-variables';
@import 'lms/static/sass/partials/lms/theme/variables-v1';
```
In the /edx/app/edxapp/lms.env.json (in Ironwood) or in /edx/etc/lms.yml (in Juniper and newer) file inside the LMS devstack, enable comprehensive theming (set ENABLE_COMPREHENSIVE_THEMING: true), add the parent directory of the cloned repo to the COMPREHENSIVE_THEME_DIRS array.
Set the value of the DEFAULT_SITE_THEME variable to the name of the cloned theme repository, say edx-simple-theme.
Save the file and close it.
Inside the LMS devstack, from the /edx/app/edxapp/edx-platform directory, run paver update_assets lms. Verify that the static assets are compiled and collected without any errors.
Restart the LMS container to apply the changes made to lms.env.json.
Go to http://localhost:18000/courses and see that the course listing item looks like this now:

open-craft / edx-simple-theme

[BB-2882] Add `main-color` to course info #14