Added the Follower model.
Added the Follower model.

This will be used to let people subscribe to for instance a student proposal so that they can receive updates about it.

Patch by: Lennard de Rijk
Reviewed by: to-be-reviewed

MatthewWilkes/mw4068-packaging,MatthewWilkes/mw4068-packaging,MatthewWilkes/mw4068-packaging,MatthewWilkes/mw4068-packaging