Add basic, rough support for changing field behavior based on object subtype

Summary:
Ref T13248. This will probably need quite a bit of refinement, but we can reasonably allow subtype definitions to adjust custom field behavior.

Some places where we use fields are global, and always need to show all the fields. For example, on /maniphest/, where you can search across all tasks, you need to be able to search across all fields that are present on any task.

Likewise, if you "export" a bunch of tasks into a spreadsheet, we need to have columns for every field.

However, when you're clearly in the scope of a particular task (like viewing or editing T123), there's no reason we can't hide fields based on the task subtype.

To start with, allow subtypes to override "disabled" and "name" for custom fields.

Test Plan:

• Defined several custom fields and several subtypes.
• Disabled/renamed some fields for some subtypes.
• Viewed/edited tasks of different subtypes, got desired field behavior.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13248

Differential Revision: https://secure.phabricator.com/D20161