Allow workboard background colors to be configured

Summary:
Adds a UI for selecting background colors.

You can choose "Use Parent", which is the default, and allows you to set a color that all descendants inherit.

You can also choose "None", if a parent has a WHACKY BACKGROUND that you refuse to put up with.

Test Plan:

Reviewers: chad

Reviewed By: chad

Differential Revision: https://secure.phabricator.com/D15279