Reruns/cms server side LMS-11016

Perhaps the standard comment about schema migration should go at the top of this file? For example:
```
If you make changes to this model, be sure to create an appropriate migration
file and check it in at the same time as your model changes. To do that,

1. Go to the edx-platform dir
2. ./manage.py lms schemamigration bulk_email --auto description_of_your_change
3. Add the migration file created in edx-platform/lms/djangoapps/bulk_email/migrations/
```