Fix old image macros and memes for logged out users

Summary:
Fixes T6013. Old image macros/memes never had the file edge written.

We also never wrote file edges for audio.

Finally, the meme controller didn't allow public access.

Write edges for images and audio, perform a migration to populate the historic ones, and make the Editor keep them up to date going forward.

Test Plan:
  - Updated image, saw new image attach and old image detach.
  - Updated audio, saw new audio attach and old audio detach.
  - Ran migration.
  - Viewed memes as a logged-out user.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T6013

Differential Revision: https://secure.phabricator.com/D10411

src/applications/macro/controller/PhabricatorMacroMemeController.php

@@ -3,6 +3,10 @@
 final class PhabricatorMacroMemeController
   extends PhabricatorMacroController {
 
+  public function shouldAllowPublic() {
+    return true;
+  }
+
   public function processRequest() {
     $request = $this->getRequest();
     $macro_name = $request->getStr('macro');