Notebook text editor

This is a work in progress, but basic loading, editing and saving appears to be working.

This sits on top of #6865, because `CodeMirror.findModeByMIME` is apparently new in 4.7.

closes #1581
I think there are a lot of imports here that you don't use,
-namespace
-jquery
-utils
-dialog
-moment
I'm guessing these are left overs from copying the base class's code.