Security callback to limit question pool access to pool pool members

Two new security callbacks for the sites configuration should be added:

1) true for members of a private question pool (no author check)
2) true for members of a private question pool and all authors

both callback should also be true for sys admins and pool admins

In some situations the teachers do not have authoring rights but want to participate in the pool with the other authors (case 1 and 2). In others, the pool should only be visible to some of the authors but not all (case 1).

Text:
1: Poolmitglieder und Poolverwalter
2: Autoren, Poolmitglieder und Poolverwalter