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

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extra : convert_revision : svn%3A32761e7d-7263-4528-b7be-7235b26367ec/trunk%402549

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