Add motion state for internal recommendations

[jsangmeister]

extracted from #816

The requirements changed a little as opposed to what was stated in the old issue. The new situation is the following:

A motion state has the new boolean flag is_internal (renamed from is_internal_recommendation - I will change this in the backend). If a motion is in state with is_internal == true, its recommendation_id should be restricted to be only visible to users with motion.can_manage. This is independent of the recommendation!

@ostcar please reevaluate how feasible this is and, if possible, implement it.