views: refactor page titles.

Refactor `page_title` helper to automatically include site name.

app/views/dmails/index.html.erb

@@ -1,3 +1,6 @@
+<% page_title "Messages" %>
+<%= render "secondary_links" %>
+
 <div id="c-dmails">
   <div id="a-index">
     <% if params[:folder] == "sent" %>
@@ -31,9 +34,3 @@
     <%= numbered_paginator(@dmails) %>
   </div>
 </div>
-
-<%= render "secondary_links" %>
-
-<% content_for(:page_title) do %>
-  Messages - <%= Danbooru.config.app_name %>
-<% end %>