Enable code folding in the editor

[sieverssj]

We use CodeMirror today and should try to enable code folding. This would allow users to fold and hide pieces of code they don't care about.

See http://codemirror.net/demo/folding.html for more information.

[sieverssj]

The foldgutter code doesn't appear to be in a release of CodeMirror yet. I expect to see it in 3.15.0 when it comes out. Based on the past release record, this should be any time. CodeMirror releases every month with the last release being June 20.