Move column hide/show to the column context menu on workboards
Summary: Fixes T5993. Now that we have a context menu we can make some edit operations easier to access.
Test Plan: Toggled column visibility. Verified board state (columns shown/hidden, ordering) was retained.
Reviewers: btrahan
Reviewed By: btrahan
Subscribers: epriestley
Maniphest Tasks: T5993
Differential Revision: https://secure.phabricator.com/D10417