Log who and when changed a user state

A simple version would save just the username and datetime of the last change in the view https://github.com/inyokaproject/inyoka/blob/staging/inyoka/portal/views.py#L828.

In an extended version, a complete log of all changes is saved.

see https://forum.ubuntuusers.de/topic/xylit-aus-unbekannten-grund-gesperrt/ for an example case