make course advanced setting invite_only configurable site-wide
Fixes mitodl#135 and reference to previous PR# https://github.com/edx/edx-platform/pull/20596
We would like to make the ~default~ value of the invitation_required attribute of the courseware object configurable via a feature flag, to be named ~COURSE_DEFAULT_INVITE_ONLY~ COURSES_INVITE_ONLY
As it is currently, this is a manually updated flag in the course advanced settings and we would like to be able to manage the ~default~ value across all courses in a given deployment. The default value of the added setting should remain the same as the current setting of False, but allow us to set it to True through the existing settings mechanism.
We have merged this solution in our forked repository mitodl#126.