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

src/infrastructure/customfield/field/PhabricatorCustomField.php

@@ -1648,6 +1648,7 @@ abstract class PhabricatorCustomField extends Phobject {
     $subtype_roles = array(
       self::ROLE_EDITENGINE,
       self::ROLE_VIEW,
+      self::ROLE_EDIT,
     );
 
     $subtype_roles = array_fuse($subtype_roles);