R1:dc7ecf58753f

Add "subtype" storage to Maniphest tasks

Summary: Ref T12314. Provides a field on tasks for storing subtypes. Does nothing interesting yet.

Test Plan:
- Ran storage upgrade.
- Created some tasks.
- Looked in the database.
- Used Conduit to query some tasks.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12314

Differential Revision: https://secure.phabricator.com/D17441