Add course advanced setting to enable/disable the creation of a CCX on a per course level

[pdpinch]

Course authors can enable advanced features in courses using the "advanced settings" in studio, or by adding a key to their policy.json file (see the course authoring docs for the SGA XBlock as an example).

edX has asked us to add a similar configuration setting for CCX.

• We will need to create a new key in the advanced settings. The recommended key is "enable_ccx" with a label of "Enable CCX" and help text of "Allow course instructors to assign CCX Coach roles, and allow coaches to manage Custom Courses on edX"
• If the advanced setting is false/disabled, instructors should not be able to add "CCX Coach" roles via the instructor dashboard's membership tab.
• We should not disable any other access to CCX based on this setting until #43 is done, as that will likely change the necessary logic.

[pdpinch]

https://github.com/edx/edx-platform/pull/8207 was merged last week.