Revert "Don't use code tags when passing math to MathJax"

This reverts commit 2d70a6142cd2fe1b2fcb9499eb1bd7dc58c352c8.

Test plan:
Load http://localhost:8000/exercises/divisibility.html?debug and verify correct answer is accepted
Also check http://localhost:8000/exercises/ideal_stoichiometry.html?seed=83&problem=na-cl2---nacl&debug looks ok

Auditors: alpert
Merge branch 'master' of https://github.com/Khan/khan-exercises
Fix bug where there was a possibility to get duplicate numbers