Respect "disabled" custom field status granted by "subtype" configuration in form validation

Summary:
Fixes T13384. Currently, the subtype "disabled" configuration is not respected when selecting fields for ROLE_EDIT.

The only meaningful caller for ROLE_EDIT is transaction validation, but transaction validation should respect fields being disabled by subtype configuration.

Test Plan:

• Added a "required" Maniphest custom field "F", then "disabled" it in a subtype "S".
• Created a task of subtype "S".
  • Before: Form submission fails with error "F is required", even though the field is not actually visible on the form and can not be set.
  • After: Form submits cleanly and creates the task.

Maniphest Tasks: T13384

Differential Revision: https://secure.phabricator.com/D20726