[CS2] Fix CS2 docs mobile issues, improve performance

[GeoffreyBooth]

The CS2 docs are currently borderline unusable on a phone. This PR aims to fix that. In particular:

• The code examples don’t become editable until you hover over (or tap/click) them. This lets us avoid initializing a few dozen copies of CodeMirror on startup. The non-editable code still has syntax highlighting generated by CodeMirror, but it’s generated at compile time and baked into the HTML. The initialization of CodeMirror should be almost unnoticeable.
• “Try CoffeeScript” is now available on mobile.
• On smaller breakpoints, the sidebar now slides over the main text rather than pushing the main column out of the way. This is much faster for browsers to render.
• The code for the docs themselves is now compiled into JavaScript during docs compilation, not via the browser compiler when the docs load. (Previously, Safari 9 was throwing an error on some syntax we use in the docs code; and that’s too new of a browser to not support.) Having the docs code already in JavaScript (and minified) beforehand is faster anyway.
• Various fixes for mobile styling and Bootstrap 4 Beta.
• Improved handling of hashes, including avoiding /#.

Take a look: http://rawgit.com/GeoffreyBooth/coffeescript/docs-perf/docs/v2/

[lydell]

Awesome!