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

launderette/templates/launderette/launderette_book_choose.jinja

@@ -5,7 +5,7 @@
 {% endblock %}
 
 {% block content %}
-{% if request.user.is_in_group(settings.SITH_MAIN_MEMBERS_GROUP) %}
+{% if request.user.is_subscribed %}
 <ul>
     {% for l in launderette_list  %}
     <li><a href="{{ url('launderette:book_slot', launderette_id=l.id) }}">{{ l }}</a></li>

launderette/templates/launderette/launderette_main.jinja

@@ -8,7 +8,7 @@
 {% if request.user.can_edit(page) %}
 <p><a href="{{ url('core:page_edit', page_name=page.get_full_name()) }}">{% trans %}Edit presentation page{% endtrans %}</a></p>
 {% endif %}
-{% if request.user.is_in_group(settings.SITH_MAIN_MEMBERS_GROUP) %}
+{% if request.user.is_subscribed %}
 <p><a href="{{ url('launderette:book_main') }}">{% trans %}Book launderette slot{% endtrans %}</a></p>
 {% endif %}