Add a "Batch Edit Tasks..." action to workboard columns

Summary:
Ref T5523. Adds a new workflow to make some kinds of bulk workboard operations easier.

New dropdown action:

This brings you into the existing bulk edit flow:

When you save an edit, you're taken back to the board:

If you try to edit a column with nothing in it, you get an error:

Note that the selected workboard filter is applied before choosing tasks, so if your filter is set to "open tasks" we only batch edit the open (i.e., currently visible) tasks in the column. I think this is more powerful (it lets you use filtering to select task subsets) but might not be completely obvious in all cases (although I do think it's more obvious than the alternative rule -- just an issue of neither rule being completely obvious).

Test Plan:

• Batch edited tasks in a column.
• Used "Batch Edit Tasks..." to move tasks to a different workboard by removing + adding a project.
• Batch edited a column with filtered-out tasks, verified only visible tasks were edited.
• Batch edited a column with no visible tasks, received error.
• Used the batch editor normally (Maniphest -> Maniphest, no boards).

Reviewers: chad, btrahan

Reviewed By: btrahan

Subscribers: johnny-bit, cburroughs, epriestley

Projects: Prioritized

Maniphest Tasks: T5523

Differential Revision: https://secure.phabricator.com/D12475