Full user extractor

[szattila98]

Implement an extractor which instead of only getting the userId from the cookie of the request, should also query the whole user from the db.
This may not be an easy task, as some kind of caching may be needed so not every request makes a database query, where it is used (but maybe not, this needs more discussion).
This would be nice because sometimes the whole user is needed and it would spare us the code duplication.
AuthenticatedUser extractor should be renamed then to AuthenticatedUserId and this should be the AuthenticatedUser. Both should remain as both will probably be needed.