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

scripts/files/manage_files.php

@@ -17,8 +17,9 @@ $args->parseStandardArguments();
 $workflows = array(
   new PhabricatorFilesManagementEnginesWorkflow(),
   new PhabricatorFilesManagementMigrateWorkflow(),
-  new PhutilHelpArgumentWorkflow(),
   new PhabricatorFilesManagementRebuildWorkflow(),
+  new PhabricatorFilesManagementPurgeWorkflow(),
+  new PhutilHelpArgumentWorkflow(),
 );
 
 $args->parseWorkflows($workflows);