Add exceptions to the `user_loader`
Add exceptions to the `user_loader`

This has been made due to performance reasons, since else, *every time*
a media/similar file is loaded, the `user_loader` kicks in and makes at
least an LDAP call.

Fix #170 with greatly little effort.

MarauderXtreme/sipa,lukasjuhrich/sipa,lukasjuhrich/sipa,agdsn/sipa,MarauderXtreme/sipa,MarauderXtreme/sipa,agdsn/sipa,lukasjuhrich/sipa,lukasjuhrich/sipa,agdsn/sipa,agdsn/sipa