Better usage of cache for groups and clubs related operations (#634)

* 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

pedagogy/models.py

@@ -147,7 +147,7 @@ class UV(models.Model):
         """
         Can be created by superuser, root or pedagogy admin user
         """
-        return user.is_in_group(settings.SITH_GROUP_PEDAGOGY_ADMIN_ID)
+        return user.is_in_group(pk=settings.SITH_GROUP_PEDAGOGY_ADMIN_ID)
 
     def can_be_viewed_by(self, user):
         """