TNL-6815

Please note that this is a squshed commit and the work of:
Symbolist, macdiesel, nedbat, doctoryes, muzaffaryousaf and muhammad-ammar

On devops recommendation, now handling the potential for an 'inconsistency
window' via a management command instead of a hacky "re-run the data migration"
bash script.

These are the migrations needed for CohortMembership to function.
0005 establishes the table, 0006 will be used to move existing data
into the table as needed.

Per product guidance, we can just arbitrarily reassign problem users.
Implementing that decision, as well as the remainder of the data migration.

Also including a short script to re-run the 0006 migration after code changes
are live, to prevent a potential issue where the database become out-of-sync.

TNL-1268

TNL-181

JIRA: TNL-937

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.