Move `roles` off the `user` table

[seanh]

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?

[seanh]

Slack thread: https://hypothes-is.slack.com/archives/C1MA4E9B9/p1643808229962529