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.


That might actually not be enough to list all existing courses, if some have an enrollment date set in the future, or are hidden - cf a recent ML thread: https://groups.google.com/forum/#!topic/edx-code/BmTm1WEWNQE