Move `roles` off the `user` table

The `user` table includes a `rows` column (instructor, learner, admin, etc):

https://github.com/hypothesis/lms/blob/0ae07d059298b7b42c72614f1dd04e39078660fb/lms/models/user.py#L37-L38

This isn't right: the same user can be an instructor in one course and a learner in another.

We should move the `roles` column off the `user` table, probably onto the `group_membership` table.

Questions:

* Can users also have different roles in different Blackboard or Canvas groups within the same course?