Add groupmode functionalities

[tobiasreischmann]

It should be possible to restrict an instance to one group. The report should then only show all members of this group instead of everyone.
Background is the distribution of seminarslots under tutors.

[Dagefoerde]

Maybe you could handle this in a similar way to other activities:
For users with sufficient rights (Managers, Teachers, Editing Tutors, ...?), show a dropdown that allows to switch between groups, thus enabling to look into every group's views. For the others, only their group-specific view is displayed.