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