Improve Ace Modes || Switch to CodeMirror

[amilner42]

At least for Elm, the modes on github are way nicer than the modes I'm using. It'd be really nice to use the SAME modes as github.

(modes = syntax highlighting).

[amilner42]

Turns out Github used to use Ace, but they switched to CodeMirror...FUCK...

It may be worth porting to CodeMirror as well, it seems to be more popular these days and have better themes/syntax-highlighting/support....

The API can support all the same features, I double checked, in fact the API seems to be much smaller and better documented. It will probably make our lives easier down the line...