ox-html: Use upstream MathJax CDN
* ox-html.el (org-html-mathjax-options): Add multlinewidth,
autonumber, tagindent, font, linebreaks and tagside. Remove MathML.
Change default indent to correspond to upstream default. Change
default MathJax path to point to upstream CDN.
(org-html--build-mathjax-config): Remove MathML-related parts.
(org-html-mathjax-template): Simplifiy template.
* org.texi (@LaTeX{} fragments), (Math formatting in HTML export):
Reflect change in default CDN.
* ORG-NEWS: Document changes.