Revert multiple cohorts feature and add a revert management command

Following the discussion on the upstream PR: https://github.com/edx/edx-platform/pull/4364 , it looks like this feature is not needed anymore for the client need. 

This PR is the complete revert + the addition of a management command to ensure things are ok. Basically, the management script will:

* Check & Fix all CourseUserGroups that have an invalid group_type (workgroup)
* Check & Fix that users or in at least 1 cohort for a course.
* Check that users are not in multiple cohorts for a course. (This have be fixed manually using the LMS dashboard)

To use the management command, run:

```
./manage.py lms revert_workgroup 
```

You should see a output of the current issues and some instruction to fix them. Then, run the management command with the --fix option and apply the manual fixes if there are.


@aboudreault Also, cf the second part of my comment, about doing this check on the model, to ensure we won't forget the constraint somewhere else in the future.