support subresource integrity [was: CDN does not offer specific versions]

Created by: alexbecker

According to the documentation, the MathJax CDN does not offer any endpoints for specific versions, but only for latest patch versions. This makes using subresource integrity with the CDN effectively impossible. I understand the desire to give users the latest patches--this is one reason to use the CDN after all--but it should be possible to use the CDN without compromising security.