* Better usage of cache for group retrieval
* Cache clearing on object deletion or update
* replace signals by save and delete override
* add is_anonymous check in is_owned_by
Add in many is_owned_by(self, user) methods that user is not anonymous. Since many of those functions do db queries, this should reduce a little bit the load of the db.
* Stricter usage of User.is_in_group
Constrain the parameters that can be passed to the function to make sure only a str or an int can be used. Also force to explicitly specify if the group id or the group name is used.
* write test and correct bugs
* remove forgotten populate commands
* Correct test
