Add a bin/files purge workflow

Summary:
We can lose file data through various means; one reasonable way is if files get deleted from disk with 'local-disk' storage. If data goes missing,

Ref T3265. Also, reduce some code duplication.

Test Plan:
Ran `bin/files purge`, `bin/files migrate`, `bin/files rebuild` with various args.

Deleted a file with "local-disk" storage, ran `bin/files purge`, made sure it got picked up.

Reviewers: dctrwatson, btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T3265

Differential Revision: https://secure.phabricator.com/D6068

src/applications/files/storage/PhabricatorFile.php

@@ -429,7 +429,13 @@ final class PhabricatorFile extends PhabricatorFileDAO
     // If this is the only file using the storage, delete storage
     if (!$other_file) {
       $engine = $this->instantiateStorageEngine();
-      $engine->deleteFile($this->getStorageHandle());
+      try {
+        $engine->deleteFile($this->getStorageHandle());
+      } catch (Exception $ex) {
+        // In the worst case, we're leaving some data stranded in a storage
+        // engine, which is fine.
+        phlog($ex);
+      }
     }
 
     return $ret;