Add MathJax to support LaTeX

[wlwilliamx]

Hi, I love your Chalk so much. It's very elegant. I found I need LaTeX so I added MathJax to support it.

And there is an error in gtag.js, it should be in the head.html instead of the javascripts.html. Otherwise, Google Analytics will make an error when detecting gtag.js. So I move the gtag.js to the head.html.

And thanks for your works.