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/templates/core/user_clubs.jinja

@@ -67,7 +67,10 @@
     </tbody>
 </table>
 
-{% if profile.mailing_subscriptions.exists() and (profile.id == user.id or user.is_root or user.is_in_group(settings.SITH_GROUP_COM_ADMIN_ID)) %}
+{% if
+    profile.mailing_subscriptions.exists()
+    and (profile.id == user.id or user.is_root or user.is_com_admin)
+%}
     <h4>{% trans %}Subscribed mailing lists{% endtrans %}</h4>
     {% for sub in profile.mailing_subscriptions.all() %}
         <p>{{ sub.mailing.email }} <a href="{{ url('club:mailing_subscription_delete', mailing_subscription_id=sub.id) }}">{% trans %}Unsubscribe{% endtrans %}</a></p>