Differentiate between "Move Tasks to Column..." and "Move Tasks to Project..." in the workboard UI

Summary:
Depends on D20635. Ref T4900. Fixes T13316.

Currently, "Move Tasks to Column..." first prompts you to select a project, then prompts you for a column. The first step is prefilled with the current project, so the common case (moving to another column on the same board) requires you to confirm that you aren't doing an off-project move by clicking "Continue", then you can select a column.

This isn't a huge inconvenience and the workflow isn't terribly common, but it's surprising enough that it has come up a few times as a stumbling block. Particularly, we're suggesting to users that they're about to pick a column, then we're asking them to pick a project. The prompt also says "Project: XYZ", not "Project: Keep in current project" or something like that.

Smooth this out by splitting the action into two better-cued flows:

• "Move Tasks to Project..." is the current flow: pick a project, then pick a column.
  • The project selection no longer defaults to the current project, since we now expect you to usually use this flow to move tasks to a different project.
• "Move Tasks to Column..." prompts you to select a column on the same board.
  • This just skips step 1 of the workflow.
  • This now defaults to the current column, which isn't a useful selection, but is more clear.

In both cases, the action cue ("Move tasks to X...") now matches what the dialog actually asks you for ("Pick an X").

Test Plan:

• Moved tasks across projects and columns within the same project.
• Hit all (I think?) the error cases and got sensible error and recovery behavior.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13316, T4900

Differential Revision: https://secure.phabricator.com/D20636