R1:9c95b387bd1e

Tidy up Workboard CSS a little

Summary:
- Custom scrollbars, colors
- New div with some better padding (floor for the column)
- More consistent spacing around the board itself.
- Slightly darker columns
- Smaller horizonal scrollbar

Test Plan:
Chrome Mac / Desktop.

{F1100342}

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

Differential Revision: https://secure.phabricator.com/D15217