Move workboard "Bulk Edit Tasks" workflow to a separate controller

Summary: Depends on D20633. Ref T4900. Separate the "Bulk Edit Tasks..." flow out of the main workboard controller.

Test Plan:

• Used "Bulk Edit Tasks" on a column with some tasks, got an appropraite edit operation.
• Used "Bulk Edit Tasks" on an empty column, got an error.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T4900

Differential Revision: https://secure.phabricator.com/D20634