Allow applications to test if a user could edit a certain field by clicking "Edit Thing"

Summary: See D15432. There, we can use this test to check if the user could reassign the task by using "Edit Form" or the stacked actions, so any dedicated "claim" element is consistent with the other permissions.

Test Plan:

• Added a var_dump($can_reassign) after the call.
• Saw true.
• Edited the edit form, locked and disabled "Assigned To".
• Saw false.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D15433