views: refactor page titles.

Refactor `page_title` helper to automatically include site name.

app/views/sessions/new.html.erb

@@ -1,3 +1,6 @@
+<% page_title "Login" %>
+<%= render "secondary_links" %>
+
 <div id="c-sessions">
   <div id="a-new">
     <section>
@@ -15,9 +18,3 @@
       </p>
   </div>
 </div>
-
-<% content_for(:page_title) do %>
- Login - <%= Danbooru.config.app_name %>
-<% end %>
-
-<%= render "secondary_links" %>