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/groups/group.html

@@ -11,7 +11,7 @@
         {% block searchresults %}
         {% endblock %}
 
-        {% include "groups/members.html" %}
+        {% include "groups/members.html"  with group=group %}
 
         <h2 class="title is-5">Lists</h2>
         {% if not lists %}