fix white space created by header by changing nav bar classes

Description

I took out the blueprint Navbar and NavbarGroup components for divs with their own classes. I have added a class to admin.less. This allows for more control over the styling of these elements. Notably, it allows for using flex boxes instead of aligns, preventing the problem where it caused white screen on smaller screen sizes.

Motivation and Context

It fixes this issue: #252

How Has This Been Tested?

Connected a basic template Empirica app to my modified empirica-core code. Made sure it the styling did what it was supposed to do in Firefox and Chrome. Testing from WSL Ubuntu.

Screenshots (if appropriate):

Types of changes

[x] Bug fix (non-breaking change which fixes an issue)
[ ] New feature (non-breaking change which adds functionality)
[ ] Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

[x] My code follows the code style of this project.
[ ] My change requires a change to the documentation.
[ ] I have updated the documentation accordingly.
[x] I have read the CONTRIBUTING document.
[ ] I have added tests to cover my changes.
[ ] All new and existing tests passed.

I tried my best to check that it was working and that it was helpful. I am happy to hear how this might need to be tweaked further. First time I am doing a change for a big repo like this one.

empiricaly / meteor-empirica-core