UID/GID switching per-request, 403 forbidden on permission error, ...

• requires the python3-sss package to get list of groups by username
• additionally added dependency on cachetools to cache the user/group information
• fix: send 403 Forbidden on PermissionError (also for multi-responses!)
• log tracebacks on internal server errors!
• configure gunicorn
  • use unix domain socket instead of tcp/ip (hardcoded for now)
  • use sync worker class, as the uid switching is not thread safe (18 workers hardcoded for our deployment for now)
• Fixes for getlastmodified and non-standard last_modified and lastmodified properties