Make numbers mathy and fix hint colours

Summary: I made the 68 - 95 - 99.7% mathy for consistency. I've replaced orange with red and swapped spans with styles with colours in code tags. The removal of hint colours also wasn't working, maybe something to do with now using KaTeX, but it should be fixed now.

Reviewers: eater

Reviewed By: eater

Subscribers: emily

Differential Revision: http://phabricator.khanacademy.org/D12687