When editing a subtyped object, use edit forms of the same subtype

Summary:
Ref T12314. When we pick an "Edit" form for a subtyped object, only consider forms with the same subtype.

For example, editing an "Animal" uses the forms with subtype "animal" which are marked as edit forms.

This also makes "Create Subtask" carry the parent task's type.

Test Plan:

• Edited an Animal, got an animal edit form.
• Edited a normal task, got a normal task form.
• Edited a paste, got the normal workflow.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12314

Differential Revision: https://secure.phabricator.com/D17445