feat: DEFAULT_COURSE_INVITATION_ONLY allow for invitation_only new courses by default

[viadanna]

Description

This pull request adds a DEFAULT_COURSE_INVITATION_ONLY setting to the CMS that, when set to True, makes all courses created in Studio invitation-only by default.

Supporting information

• BB-8746

Testing instructions

1. Deploy this branch to a sandbox or devstack.
2. Set the DEFAULT_COURSE_INVITATION_ONLY configuration to True in the appropriate configuration file.
3. Create a new course using Studio.
4. Verify the course Advanced Settings has invitation_only=True
5. Set the DEFAULT_COURSE_INVITATION_ONLY configuration to False in the appropriate configuration file.
6. Repeat steps 3 and 4 to verify invitation_only=False.

Deadline

None

[CefBoud]

Good job on this, @viadanna ! I left one small question but LGTM ✅

• I tested this
• I read through the code
• Includes documentation (N/A)