Add a "subtype" field to EditEngine forms

Summary:
Ref T12314. This adds storage so EditEngine forms can later be marked as edit fields for particular types of objects (like an "animal edit form" vs a "plant edit form").

We'll take you to the right edit form when you click "Edit" by selecting among forms with the same subtype as the task.

This doesn't do anything very interesting on its own.

Test Plan:

• Ran bin/storage upgrade.
• Verified database got the field with proper values.
• Created a new form, checked the database.
• Ran unit tests.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12314

Differential Revision: https://secure.phabricator.com/D17439