Rough cut of "Move tasks to column..."

Summary:
Ref T5523. See PHI50. See PHI40. This isn't perfect, but should improve things.

Add a "Move tasks to column..." action to workboards which moves all visible tasks in a column to another column, either on the same board or on a different board.

This is a two-step process so that I don't have to write Javascript, and because I'm not 100% sure this is what users actually want/need. If it sticks, the UI could be refined later.

• The first dialog asks you to choose a project, defaulting ot the current project.
• The second dialog asks you to choose a column on that project's board.

Test Plan:

• Moved tasks on the same board.
• Moved tasks to a different board.
• Tried to move tasks to the starting column, got a sensible error.
• Tried to move tasks to no project, got a sensible error.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T5523

Differential Revision: https://secure.phabricator.com/D18665