"Raw HTML" contains rendered MathJax output

### Is this a hackmd.io issue?

- [x ] Yes, the bug happens on https://hackmd.io

### What's the problem
When exporting as "raw HTML" all math content is included as the mathjax output.

#### Current behaviour
The output of mathjax is included in the "raw" html. This does not do any good as the raw html does not include the css to render the mathjax spans correctly. The math just shows up as a jumbled mess.

#### Expected behaviour
Math should be included "as-is", i.e. `$$E=mc^2$$ `
If this was the behaviour, I could use the output in a context where mathjax is already loaded or could add a script tag to load mathjax and it would all render correctly.

Yes I can export the markdown and use pandoc but then I'll loose some hackmd features in the html which I like.

#### Additional context
Fixing this might resolve or simplify a solution for #118