Use codemirror package

This pull for #1655 switches to vanilla CM except for themes and will [fix these upstream issues](https://github.com/LightTable/LightTable/issues?q=is%3Aopen+label%3Acm+label%3Afixed-upstream) as well as fix the foldgutter css. This uses the 4.7.0 package since 4.8.0 loaded with errors (perhaps due to [key changes](http://codemirror.net/doc/manual.html#normalizeKeyMap)). CM addons that had been modified were moved to deploy/core/node_modules/codemirror_addons.

@ibdknox Two questions:
* Can we use CM's themes at some point? I did not switch to them because most of LT's themes had been modified.
*  The overlay.js addon we use only has one commit, 12aa9ee7cf9a91c1b4090438604a024b970daaca. This is needed, right? If so, I'll add it to the list of addons to eventually send changes upstream.

Besides these two minor questions, this should be good to go.