Filter groups in ACL selector

[LukasKalbertodt]

Olaf brought up a feature request where in the ACL selector, users are only shown some of the groups. This is especially helpful if you add one group per, say, university course, resulting in tons of possible groups. The formulated feature was to "only see groups related to the course the user is in". Of course that's pretty hard.

But I think the following model can work:

• Users only see groups that they are part of (i.e. the group roles they have)
• Some groups can be marked as global or public or something so that they can always be selected, regardless of whether the user is part of it (useful for, "academic guests" or something)

[oas777]

Thanks, Lukas. For the time being, the first of the two should be sufficient for us.