R1:0561043a1f57

In "Move task to..." workflow, separate visible and hidden columns in the dropdown

Summary:
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.

Test Plan:
- Used "Move tasks to project..." targeting a board with no hidden columns. Saw a single ungrouped dropdown.
- Used "Move tasks to project..."…