Don't run MathJax on hints immediately

Summary: It's silly and slow to run MathJax on the hints which might not even been seen! Instead, move the MathJax-calling out to a separate module which (like graphie) is called when the content is actually presented. This should also be a bit faster in all cases because we only call MathJax once for the question area and once per hint instead of once for each `<code>` tag.

Test Plan:
reading_stem_and_leaf_plots.html feels faster; hints still appear correctly.
multiplying_expressions_0.5.html choices look the same; grading (including none of the above) works.
addition_1.html acceptable formats works.
graphing_linear_equations.html looks sane too.

Reviewers: eater, emily

Reviewed By: emily

CC: ben

Differential Revision: http://phabricator.khanacademy.org/D2967