Add maniphest.subtypes for configuring task subtypes

Summary:
Ref T12314. Builds toward letting you define "animal" and "plant" tasks.

This just adds some configuration. I'll probably add some more quality-of-life options (like "icon") later but these are the only bits I'm sure I'll need.

Test Plan:

• Configured sensible subtypes.
• Tried to configure bad subtypes: bad key, missing "default", duplicate keys. Got sensible error messages.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12314

Differential Revision: https://secure.phabricator.com/D17440