Added light color alternatives to existing KhanUtil colors

Summary:
The Plotter widget uses a light blue, so I added light alternatives
to the other existing KhanUtil colors for use by content creators. These new
colors were chosen by taking the hue of the dark analog and modifying its
saturation and lightness to mirror the relationship between the existing blue
and the light blue used by the Plotter.

Differential Revision: http://phabricator.khanacademy.org/D9718