R1:8a7c96390862

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…