Move workboard "Move Tasks to Column..." workflow to a separate controller

Summary: Depends on D20634. Ref T4900. Ref T13316. I'm planning to do a bit of additional cleanup here in followups, but this separates the main workflow out of the common controller.

Test Plan:

• Used "Move Tasks to Column..." to move some tasks on a board.
• Tried to move an empty column, hit an error.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13316, T4900

Differential Revision: https://secure.phabricator.com/D20635