#147 - Limit number of courses and invites

https://trello.com/c/3TQ9inqE/147-only-allow-the-user-to-invite-one-user-to-their-course-unless-they-re-on-a-class-department-or-college-plan
https://trello.com/c/6l3U0XId/71-limit-how-many-courses-a-user-can-create

Not yet final, sent a pull request so I can see the changes more easily.