HTML template for exercise summary

Summary: CSS styles for exercise summary - mostly graph colours

Test Plan:
Open dashboards.khanacademy.org and check exercise-summary dashboard

On exercise-summary console should not show any less related errors.
Style should be consistent with the screenshot on the main page.

Reviewers: jace, chris

Reviewed By: chris

Differential Revision: http://phabricator.khanacademy.org/D2230