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.

Reviewers: epriestley

Reviewed By: epriestley

Subscribers: Korvin

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