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_edit.jinja

@@ -49,7 +49,7 @@
     <p>{{ form.club.errors }}<label for="{{ form.club.name }}">{{ form.club.label }}</label> {{ form.club }}</p>
     <p>{{ form.summary.errors }}<label for="{{ form.summary.name }}">{{ form.summary.label }}</label> {{ form.summary }}</p>
     <p>{{ form.content.errors }}<label for="{{ form.content.name }}">{{ form.content.label }}</label> {{ form.content }}</p>
-    {% if user.is_in_group(settings.SITH_GROUP_COM_ADMIN_ID) %}
+    {% if user.is_com_admin %}
     <p>{{ form.automoderation.errors }}<label for="{{ form.automoderation.name }}">{{ form.automoderation.label }}</label>
         {{ form.automoderation }}</p>
     {% endif %}