Build basic infrastructure for an activity feed

Summary: This defines an extremely basic version of an activity feed, like
Facebook's news feed. It doesn't do much of interest yet.
Test Plan: Published some feed stories:
https://secure.phabricator.com/file/view/PHID-FILE-5061aa72105bbdc05b21/
Reviewed By: tuomaspelkonen
Reviewers: jungejason, tuomaspelkonen, aran
Commenters: codeblock, jungejason
CC: aran, epriestley, codeblock, tuomaspelkonen, jungejason
Differential Revision: 593

src/aphront/default/configuration/AphrontDefaultApplicationConfiguration.php

@@ -330,6 +330,10 @@ class AphrontDefaultApplicationConfiguration
         'delete/(?P<id>\d+)/$'
           => 'PhabricatorCountdownDeleteController'
       ),
+
+      '/feed/' => array(
+        '$' => 'PhabricatorFeedStreamController',
+      ),
     );
   }