Only prevent drag-scroll in the Y direction on workboards

Summary: This scroll lock thing prevented both X and Y scrolling, but should only prevent Y scrolling. Dragging a card to the edge to scroll left/right is fine.

Test Plan: Scrolled a workboard left/right by dragging a card to the edge.

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D15213