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

forum/models.py

@@ -161,7 +161,9 @@ class Forum(models.Model):
     # divided by 3 the number of requests on the main forum page
     # after the first load
     def is_owned_by(self, user):
-        if user.is_in_group(settings.SITH_GROUP_FORUM_ADMIN_ID):
+        if user.is_anonymous:
+            return False
+        if user.is_in_group(pk=settings.SITH_GROUP_FORUM_ADMIN_ID):
             return True
         try:
             m = Forum._club_memberships[self.id][user.id]
@@ -337,7 +339,10 @@ class ForumMessage(models.Model):
     def is_last_in_topic(self):
         return bool(self.id == self.topic.messages.order_by("date").last().id)
 
-    def is_owned_by(self, user):  # Anyone can create a topic: it's better to
+    def is_owned_by(self, user):
+        if user.is_anonymous:
+            return False
+        # Anyone can create a topic: it's better to
         # check the rights at the forum level, since it's more controlled
         return self.topic.forum.is_owned_by(user) or user.id == self.author.id