Fix selection colors in main and inline editors

Restores the original blue selection color in both main and inline editors. As part of this, factored out the selection colors into LESS variables.

While fixing this, I noticed and fixed a few other issues:
* Focused inline editor selections were no longer blue--this was because `.CodeMirror-focused` is on the wrapper (I think it used to be somewhere nested inside the wrapper).
* The cursor was visible in unfocused inline editors--this was because `.CodeMirror-cursor` is no longer a `pre`.
* Consolidated a couple of rules for `.CodeMirror-cursor` (one of which I had recently introduced without noticing the other rule).