dmails: add unread folder, show only received messages by default.

* Add unread and deleted dmail folders.
* Remove dmail_folder cookie (wasn't used).
* Default to the received folder so that we don't show sent messages by default.

app/views/dmails/index.html.erb

@@ -3,10 +3,8 @@
 
 <div id="c-dmails">
   <div id="a-index">
-    <% if params[:folder] == "sent" %>
-      <h1>Sent Messages</h1>
-    <% elsif params[:folder] == "received" %>
-      <h1>Received Messages</h1>
+    <% if params[:search][:folder].present? %>
+      <h1><%= params[:search][:folder].titleize %> Messages</h1>
     <% else %>
       <h1>Messages</h1>
     <% end %>