If the user cancels a workboard drop flow, put things back where they were

Summary:
Ref T13074. If you hit a prompt on a drop operation (today: MFA; in the future, maybe "add a comment" or "assign this task"), we currently leave the board in a bad semi-frozen state if you cancel the workflow by pressing "Cancel" on the dialog.

Instead, put things back the way they were.

Test Plan: Dragged an MFA-required card, cancelled the MFA prompt, got a functional board instead of a semi-frozen board I needed to reload.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13074

Differential Revision: https://secure.phabricator.com/D20305