When an object which supports subtypes is created, set its subtype to the creating form's subtype

Summary:
Ref T12314. If you set a form to have the "plant" subtype, then create a task with it, save "plant" as the task subtype.

For Conduit, the default subtype is used by default, but a new "subtype" transaction is exposed. You can apply this transaction at create time to create an object of a certain subtype, or at any later time to change the subtype of an object.

This still doesn't do anything particularly useful or interesting.

Test Plan:

• Created a non-subtyped object (a Paste).
• Created "task" and "plant" tasks via different forms.
• Created "default" and "plant" tasks via Conduit.
• Changed the subtype of a task via Conduit.
• Tried to set a bad subtype.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12314

Differential Revision: https://secure.phabricator.com/D17443