Merge remote-tracking branch 'origin/develop'

Merge pull request mathjax#1925 from mathjax/develop

MathJax v2.7.3

Merge pull request mathjax#1827 from mathjax/develop

Release 2.7.2

Repack and recombine

Merge branch 'develop' into master for 2.7.2-beta.1 release

Merge branch 'patch-a11y'

Merge pull request mathjax#1740 from dpvc/2.7.1-rc.1

Update for 2.7.1

Merge pull request mathjax#1638 from mathjax/develop

v2.7.0 release

Update Contrib to always use https, and repack MathJax.js

Merge pull request mathjax#1382 from mathjax/develop

Merge develop into master for 2.6.1 release