Co-authored-by: Jonathan Weth <git@jonathanweth.de>

Allow operations on persons and person-related objects only if they are a member of a group the editor owns

Allow the owner of a parent group to be able to perform the same operations on the related object as the owners of all other groups the object is a member of

Needed some extra/rewritten API

Includes all related stuff like permissions, urls, forms and preferences

- My groups
- My students
- Links in week view