Support "Subtype" in Herald

Summary: See PHI1434. For objects that support subtypes and have subtypes configured, allow Herald rules to act on subtypes.

Test Plan:

• Configured task and project subtypes, wrote Herald rules, saw "Subtypes" as an option, saw appropriate typeahead values and detail page rendering.
• Unconfigured project subtypes, saw field vanish from UI for new rules.
• Wrote a "subtype"-depenent rule that added a comment, interacted with tasks of that subtype and a different subtype. Saw Herald act only on tasks with the correct subtype.

Differential Revision: https://secure.phabricator.com/D20809