Define who is allowed to collaborate on a course

[Rahbaran]

As a course creator I want to add collaborators to a created course of mine so we can work together on my created course.

1. Define the user who created a new course, as a course creator. Only this person can edit the course.
2. Let her / him add other users as collaborators who can now edit the course, too.

[Rahbaran]

I also put it in the ready col on waffle: https://waffle.io/Crowducate/crowducate-platform