Adding support for multiple teachers in a classroom & setting up RBAC for individual classrooms

While reviewing this PR (https://github.com/freeCodeCamp/classroom/pull/252), we noticed some significant areas for improvement. However, the PR is so old, that we figured it makes more sense to log the changes as a new issue.

Work on this was started here: https://github.com/freeCodeCamp/classroom/pull/252

The contributors ended up doing some migrations that looked awkward/hacky. After changing the teacherId to an array, the UserID foreign key constraint broke. They tried to fix the issue by making the foreign key constraint optional. A better solution is to create a new role (classroomOwner). The previous UserID foreign key should refer to the classroomOwner's Id.

Eventually, we also want to be able to use this foundation to set up RBAC inside a classroom. (Hence the need for a classroomOwner role)