fix group references in templates

Let's do this the sensible way huh, by using backwards references to memberships etc

Also adds filters for is_member and is_invited so we don't have to do weird things in group Views

bookwyrm/templates/user/groups.html

@@ -24,7 +24,7 @@
 
 {% block panel %}
 <section class="block">
-    <form name="create-group" method="post" action="{% url 'user-groups' 'empty_arg' %}" class="box is-hidden" id="create_group">
+    <form name="create-group" method="post" action="{% url 'user-groups' request.user.name %}" class="box is-hidden" id="create_group">
         <header class="columns">
             <h3 class="title column">{% trans "Create group" %}</h3>
             <div class="column is-narrow">