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

doc/TW_Skia/Rapport.tex

@@ -342,7 +342,7 @@ tous les autres.
 {% if user.is_in_group(settings.SITH_GROUP_ACCOUNTING_ADMIN_ID) %}
     <li><a href="{{ url('accounting:bank_list') }}">Accounting</a></li>
 {% endif %}
-{% if user.is_in_group(settings.SITH_MAIN_BOARD_GROUP) or user.is_root %}
+{% if user.is_board_member or user.is_root %}
     <li><a href="{{ url('subscription:subscription') }}">Subscriptions</a></li>
     <li><a href="{{ url('counter:admin_list') }}">Counters management</a></li>
 {% endif %}