Upgrade CodeMirror to 4.6

The version of CodeMirror we were using in Sandcastle was extremely out of date.  This updates from 2.24 to 4.6, which included many breaking API changes (hopefully I caught them all).  Our hacked up `Cesium-Hint.js` is no longer needed and was deleted as well.

@emackey can you please put this through its paces make sure I didn't break any features I might have not known about and take a look at any other tweaks you might want to make.