Use MathJax 2.4 hosted on cdnjs.

MathJax is a huge dependency. In `edx-platform`, it is loaded from an S3 bucket hosted by edX (see
https://github.com/edx/edx-platform/blob/bf8183f61e95f655cf4f13d2ee02c0c2c8871a4d/common/templates/mathjax_include.html#L36),
but the version used in `edx-platform` is currently 2.2, which has some IE bugs that break the discussion layout, so we can't use that.
The bugs have been fixed in MathJax 2.4.