Generate mail command documentation from active commands

Summary: Ref T7199. This needs some polish and isn't reachable from the UI, but technically has all of the information.

Test Plan:
{F355899}

{F355900}

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7199

Differential Revision: https://secure.phabricator.com/D12241

src/applications/base/PhabricatorApplication.php

@@ -189,6 +189,10 @@ abstract class PhabricatorApplication implements PhabricatorPolicyInterface {
     return array();
   }
 
+  public function getMailCommandObjects() {
+    return array();
+  }
+
 
 /* -(  URI Routing  )-------------------------------------------------------- */