JIRA: TNL-710

IMPORTANT: this commit converts the course_groups
package to using migrations.  When deploying to an
existing openedx instance, migration 0001 may fail
with an error indicating that the CourseUserGroup
table already exists.  If this happens, running
the 0001 migration first, with the --fake option,
is recommended.  After performing this step,
remaining migrations should work as expected.

Previously, the user endpoint would would return 404 for any request
involving a user having an enrollment for a course that no longer
existed.

This new endpoint is designed specifically to fill the needs of the
notifier and should not be used by other clients.