In "Move Tasks to Column...", show only visible columns

Summary:
See PHI94. I considered this initially but wasn't sure about it. However, PHI94 brings up the good point that we already use a similar rule in Maniphest.

For consistency, only show visible columns here too.

Test Plan: Used "Move tasks to column..." on a board with visible and hidden columns, only saw visbile columns offered in the dropdown.

Reviewers: amckinley

Reviewed By: amckinley

Differential Revision: https://secure.phabricator.com/D18668