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