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.

plugins/messages/app/views/messagegroups/_messagegroup.html.haml

@@ -0,0 +1,8 @@
+%section.well
+  %h3= messagegroup.name
+  = render :partial => 'shared/group', :locals => { :group => messagegroup }
+  = link_to_new_message(message_params: {group_id: messagegroup.id})
+  - if messagegroup.member?(current_user)
+    = link_to t('.leave'), leave_messagegroup_path(messagegroup), class: 'btn', method: :post
+  - else
+    = link_to t('.join'), join_messagegroup_path(messagegroup), class: 'btn', method: :post

plugins/messages/app/views/messagegroups/index.html.haml

@@ -0,0 +1,5 @@
+- title t('.title')
+
+= t('.body').html_safe
+
+= render :partial => "messagegroup", :collection => @messagegroups