Use CodeMirror from a CDN

A CodeMirror clone can instead just load from a known CDN location for that code.

If the polyfill could just get inlined in the repo (maybe just copying in the couple files that are used?), and maybe even move the main branch to gh-pages, then all clones could automatically/easily go to GitHub Pages.

Then rename `history` to `commandHistory` – it seems like something happens where `window.history` conflicts with this variable on Firefox (but not Chrome).  Might also be a polyfill weirdness?  Anyway, rename fixes it.  Also a initMouseEvent fix for Firefox.

Lastly, the actual point of the changes, TogetherJS support, for collaborative logoing.