Add messagegroup

Messagegroups are a new kind of group. Every user can join and leave such a
group on her own. They are useful to create topics for mails.

E.g. if there is a weekly newsletter we can create a new messagegroup for
it and every user which is interested in receiving the newsletter can join
the corresponding group.

app/views/shared/_group.html.haml

@@ -6,8 +6,9 @@
     %dd=h group.contact
     %dt= heading_helper(Ordergroup, :contact_address) + ':'
     %dd= link_to_gmaps group.contact_address
-  %dt= t('.access') + ':'
-  %dd= format_roles(group)
+  - if group.is_a?(Workgroup)
+    %dt= t('.access') + ':'
+    %dd= format_roles(group)
   %dt= heading_helper(Ordergroup, :user_tokens) + ':'
   %dd
     - members = group.users