Rework label generation for all actors.

Some adjustments to refactor the actor labels in `opengever.core` (see https://github.com/4teamwork/opengever.core/pull/291).

* Add prefix_label to admin unit.
* User label: added `with_principal` option to label.
* Added `fetch_group` method to the OGDSService, to fetch groups by id.

@deiferni