Try to load user lists also for non-logged in users

The backend will decide whether a client is allowed to see this. The
anonymous user can be assigned permissions to be able to see these. By
default, the anonymous user isn't able to see user lists, though.
Selection table: be more robust if user lists don't exist

If a user doesn't exist a questionmark is shown instead.
Don't let user update user profile of anonymous user

Due to making the anonymous user's is_anonymous() method return false recently,
the current check didn't work anymore. Now one cannot change a user profile if a
user is either not logged in or Django's anonymous user.
Remove debug output
Tests: add more can_browse API tests
Graph export: add check id skeleton IDs have been provided
NeuroML export: only requre can_browse permissions on the project
Tests: reorganize permission test and handle non-JSON responses