In "Move task to..." workflow, separate visible and hidden columns in the dropdown
Ref T13368. Currently, both visible and hidden columns are shown in the "Move tasks to..." dropdown on workflows from workboards.
When the dropdown contains hidden columns, move them to a separate section to make it clear that they're not likely targets.
- Used "Move tasks to project..." targeting a board with no hidden columns. Saw a single ungrouped dropdown.
- Used "Move tasks to project..." targeting a board with hidden columns. Saw a dropdown grouped into "Visible" and "Hidden" columns.
Maniphest Tasks: T13368
Differential Revision: https://secure.phabricator.com/D20700