MathJax 2.3 upgrade and loading improvement

- upgrade to MathJax 2.3, use the unpacked version and remove some unusued files.
- use MediaWiki's resource loader for Javascript files.
- improve the way MathJax is configured.
- enable server side generated MathML input.
- make math fonts available as Web fonts for Gecko browsers.

Change-Id: I365367d6df00c482bed5c6b9759eece12aff7d47