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