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

com/templates/com/news_detail.jinja

@@ -35,7 +35,7 @@
         <p>{% trans %}Author: {% endtrans %}{{ user_profile_link(news.author) }}</p>
         {% if news.moderator %}
         <p>{% trans %}Moderator: {% endtrans %}{{ user_profile_link(news.moderator) }}</p>
-        {% elif user.is_in_group(settings.SITH_GROUP_COM_ADMIN_ID) %}
+        {% elif user.is_com_admin %}
         <p> <a href="{{ url('com:news_moderate', news_id=news.id) }}">{% trans %}Moderate{% endtrans %}</a></p>
         {% endif %}
         {% if user.can_edit(news) %}