Allow tasks to be searched by subtype

Summary:
Ref T12314. Allow tasks to be queried by subtype using a typeahead.

Open to a better default icon. I'll probably let you configure them later.

Just hide this constraint if there's only one subtype.

Test Plan:

• Searched for subtypes.
• Verified that the control hides if there is only one subtype.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12314

Differential Revision: https://secure.phabricator.com/D17444