Include less data in request logging
Include less data in request logging

This fixes two problems:

1. When required, the username is added to the request log just by
   accessing the `session` and not `current_user`, because the latter
   triggers the `user_loader`, which causes gathering every user
   information from the pycroft API.
2. Even more radically, we stop adding any extra information for
   `/static` endpoints, entirely.  Information like the IP is
   necessary for functional debugging (because it may be used as a
   substitute to the session for identifying the user, e.g. when using
   the traffic API), but completely pointless here.

Fixes #439

agdsn/sipa,agdsn/sipa,agdsn/sipa,agdsn/sipa