Allow object subtypes to be changed via bulk editor

Summary:
Ref T13151. See PHI683. Ref T12314.

You can currently change object subtypes via Conduit (`maniphest.edit`) but not via the web UI.

Changing object subtypes is inherently a somewhat-perilous operation that likely has a lot of rough edges we'll need to smooth over eventually, mostly around changing an object from subtype X to subtype Y, where some field exists on one but not the other. This isn't a huge issue, just not entirely intuitive.

It should also, in theory, be fairly rare.

As a reasonable middle ground, provide web UI access via the bulk editor. This makes it possible, but doesn't clutter the UI up with a rarely-used option with rough edges.

Test Plan:
  - With subtypes not configured, saw a normal bulk editor with no new option.
  - With subtypes configured, swapped tasks subtypes via bulk editor.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13151, T12314

Differential Revision: https://secure.phabricator.com/D19490

src/applications/transactions/engineextension/PhabricatorSubtypeEditEngineExtension.php

@@ -47,6 +47,11 @@ final class PhabricatorSubtypeEditEngineExtension
       ->setValue($object->getEditEngineSubtype())
       ->setOptions($options);
 
+    // If subtypes are configured, enable changing them from the bulk editor.
+    if (count($map) > 1) {
+      $subtype_field->setBulkEditLabel(pht('Change subtype to'));
+    }
+
     return array(
       $subtype_field,
     );