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

Attention: Still includes some tools to measure time load!

Needed some extra/rewritten API

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

Closes #126 — module loading must not have side effects, and
generating the predicates calls the database.

Under normal operation, cache_memoize and/or django-cachalot ensure
the database lookup is not done on every permission check.

Allow filtering of personal notes displayed in overview of last lesson and improve personal notes rules