[cmv3] Decouple horiz scrolling in inline text editors from outer editor.

@jasonsanjose 

Our original scheme for tying horizontal scrolling in inline editors to the outer editor, which involved just letting the inline editor take up its natural width, won't work with the new CodeMirror because of the fixed gutter; we need the inline editor to handle its own horizontal scrolling.

Luckily, CodeMirror v3 has an option for inline widgets not to horizontally scroll with the parent. So we simply turn that on, which makes the nested editor's client width fit into the parent's client width, and then let it scroll itself.

This also means that we can get rid of all the weird stuff we were doing with the rule list container; it can now just be a float-right child of the widget, with the nested editor fitting to its left. This gets rid of a whole bunch of logic, and as a side-effect, it seems that typing in inline editors is much faster now.

I'll add notes on some of the deleted code to indicate why it's no longer necessary.