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.com/D20273