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/admin/messagegroups/show.html.haml

@@ -0,0 +1,6 @@
+- title t '.title', name: @messagegroup.name
+
+%section= render 'shared/group', group: @messagegroup
+= link_to t('ui.edit'), edit_admin_messagegroup_path(@messagegroup), class: 'btn'
+= link_to t('ui.delete'), [:admin, @messagegroup], :data => {:confirm => t('.confirm')}, :method => :delete, class: 'btn btn-danger'
+= link_to t('.send_message'), new_message_path(:message => {:group_id => @messagegroup.id}), class: 'btn'