R1:0b2d25778d8f

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…