Don't show proxy (subproject/milestone) columns as options in "Move tasks..." workflows from workboards

Summary:
Ref T13368. Proxy columns should not be selectable from this workflow. If you want to move tasks to milestone/subproject X, do "Move tasks to project..." and pick X as the project.

(This could be made to work some day.)

Test Plan: Went through a "Move tasks to project..." workflow targeting a project with subprojects. No longer saw subproject columns presented as dropdown options.

Maniphest Tasks: T13368

Differential Revision: https://secure.phabricator.com/D20699