Popout inbox

Rather than opening the inbox in its own window, let it appear as a
browser action popup by default, but allow promotion to its own window
if requested.

background.html

@@ -163,6 +163,7 @@
   <script type="text/x-tmpl-mustache" id="inbox">
     <div class='title-bar' id='header'>
       <div class='socket-status'></div>
+      <button class='popout hide'></button>
     </div>
     <div class='gutter'>
         <div class='conversations scrollable'></div>