Add maniphest.subtypes for configuring task subtypes
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.
- Configured sensible subtypes.
- Tried to configure bad subtypes: bad key, missing "default", duplicate keys. Got sensible error messages.
Reviewed By: chad
Maniphest Tasks: T12314
Differential Revision: https://secure.phabricator.com/D17440