Allow Mathjax(Latex) in Student responses

Currently, Latex rendering is only supported for question prompts in ORA2. This
commit introduces Latex rendering in ORA2 student response, peer assessment
view, self assessment view, grade view, and staff-info view. The reason that
this Latex rendering code is inserted directly into ORA2 is that edx-platform,
does not know when to start Latex rendering for ORA2 views that come in via
AJAX.

This commit introduces "Preview in Latex" feature which allows students to see
their response rendered in MathJax before submission.

Conflicts:
	openassessment/xblock/static/js/openassessment-lms.min.js
	openassessment/xblock/static/js/openassessment-studio.min.js
fixed tests
Some quick cleanup on Javascript and tests.
Merge pull request #619 from edx/sanchez/mathjax

MathJax Integration