Allow EditEngine forms for objects which support subtyping to have a subtype configured

Summary:
Ref T12314. This adds a "Change Form Subtype" workflow to the EditEngine form configuration screen, for forms that edit/create objects which support subtyping (for now, only tasks).

For example, this allows you to switch a form from being a "task" form to a "plant" or "animal" form.

Doing this doesn't yet do anything useful or interesting. I'm also not showing it in the UI yet since I'm not sure what we should make that look like (presumably, we should just echo whatever UI we end up with on tasks).

Test Plan:

• Changed the subtype of a task form.
• Verified that the "Change Subtype" action doesn't appear on other forms (for example, those for Pastes).

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12314

Differential Revision: https://secure.phabricator.com/D17442