Allow workboards to be panned horizontally by dragging the background

Summary:
Ref T5240. For boards with a lot of columns and users without "shift + mousewheel" or a touchpad, allow click-drag on the board background to pan the board horizontally.

The ew-resize cursor cue might be a little too intense. If it's annoying, we could drop it and just leave this as a secret feature to discover.

Test Plan: Panned the board horizontally.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T5240

Differential Revision: https://secure.phabricator.com/D15211