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/user_groups.html

@@ -3,7 +3,7 @@
 {% load interaction %}
 
 <div class="columns is-multiline">
-    {% for group in user.bookwyrm_groups.all %}
+    {% for group in groups %}
     <div class="column is-one-quarter">
         <div class="card is-stretchable">
             <header class="card-header">