refactor controller/markdownToHtml.js module

authored by @Dhananjay-JSR this PR replaces https://github.com/pelias/api/pull/1598.

this PR makes several fixes:

reformat the output to serve valid HTML
use template literals to make the markdown and html code more legible.
use a standard HTML5 boilerplate with a DOCTYPE, TITLE etc.
remove the styleString variable and instead just write the style info inline in the <head><style>
clean up the controller code, removing the content negotiation code which wasn't being used
fix/clean up the unit test code.

Thanks to @Dhananjay-JSR for this contribution 🎉

pelias / api