Add more colour-coding to hints

Summary:
I've also changed the solution data-type to expression when a value with pi needs to be added because there's a number of issues where people have written answers with parentheses which doesn't work: https://khanacademy.atlassian.net/browse/AI-121148
It seems like using expression entry is the easiest way to solve this, but maybe we don't want to do that for some reason.

Reviewers: eater, emily

Reviewed By: emily

Differential Revision: http://phabricator.khanacademy.org/D13464