Add message threads #394

This new view sorts the messages by threads based on the reply_to.
Now we store only the message which started a new thread in the reply_to
field to avoid recursive queries.

plugins/messages/app/views/message_threads/show.haml

@@ -0,0 +1,16 @@
+- if @messages.first.group
+  - title @messages.first.subject + ' (' + @messages.first.group.name + ')'
+- else
+  - title @messages.first.subject
+
+- for message in @messages
+  .panel.panel-default{:style => "width:40em"}
+    .panel-heading
+      %b= h(message.sender_name)
+      = format_time(message.created_at)
+    .panel-body= simple_format(h(message.body))
+
+%p
+  = link_to t('.reply'), new_message_path(:message => {:reply_to => @messages.first.id}), class: 'btn'
+  |
+  = link_to t('.all_message_threads'), message_threads_path