Provide a public view of feed

Summary: Depends on D628. Provides a config option so you can set up a public
feed, which you can iframe. This needs some work but sort of works.
Test Plan: Loaded the public feed as a logged-out user.
Reviewed By: codeblock
Reviewers: jungejason, tuomaspelkonen, aran, codeblock
CC: aran, codeblock
Differential Revision: 635

src/aphront/default/configuration/AphrontDefaultApplicationConfiguration.php

@@ -332,6 +332,7 @@ class AphrontDefaultApplicationConfiguration
 
       '/feed/' => array(
         '$' => 'PhabricatorFeedStreamController',
+        'public/$' => 'PhabricatorFeedPublicStreamController',
       ),
 
       '/V(?P<id>\d+)$'  => 'PhabricatorSlowvotePollController',