CodeMirror has had a bunch of releases since we last updated. Most of

these deal with individual modes, but there have been some good bugfixes

that we want to pick up. This change updates us to the latest released

version, 5.48.2. As part of this I've updated the instructions for

building the file to cope with changes in their tree (the removal of the

compress script) and switched us to bundling an un-minified file,

which makes things easier for debugging.

Smoke tested various text editors and different features of the editor.