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

core/signals.py

@@ -0,0 +1,17 @@
+from django.core.cache import cache
+from django.db.models.signals import m2m_changed
+from django.dispatch import receiver
+
+from core.models import User
+
+
+@receiver(m2m_changed, sender=User.groups.through, dispatch_uid="user_groups_changed")
+def user_groups_changed(sender, instance: User, **kwargs):
+    """
+    Clear the cached clubs of the user
+    """
+    # As a m2m relationship doesn't live within the model
+    # but rather on an intermediary table, there is no
+    # model method to override, meaning we must use
+    # a signal to invalidate the cache when a user is removed from a club
+    cache.delete(f"user_{instance.id}_groups")