Allow files to TTL and and be garbage collected

Summary: Added ttl field to files. Gabage collect files with expired ttl

Test Plan: created file with a ttl. Let garbage collector run

Reviewers: epriestley

CC: aran, Korvin

Differential Revision: https://secure.phabricator.com/D4987

src/applications/diffusion/controller/DiffusionBrowseFileController.php

@@ -779,6 +779,7 @@ final class DiffusionBrowseFileController extends DiffusionController {
       $data,
       array(
         'name' => basename($path),
+        'ttl' => time() + 60 * 60 * 24,
       ));
   }