R1:74de153e59be

Allow MFA task edits to go through on workboards

Summary: Depends on D20272. Ref T13074. When a task requires MFA to edit, you currently get a fatal. Provide a cancel URI so the prompt works and the edit can go through.

Test Plan:
- Locked a task, dragged it on a workboard.
- Before: fatal trying to build an MFA gate.
- After: got MFA gated, answered prompt, action went through.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13074

Differential Revision: https://secure.phabricator.