editor progress

- load/store codemirror config
- fill out menus a bit:
  - Edit/find,replace
  - View/toggle line numbers
track dirty state in editor for onbeforeunload
basic LESS for editor
allow selecting codemirror keymap in menu
link to /edit/ on files
add File/New in editor
fixup
fixup keyMap
setting an option to null sets the default in CodeMirror

matching the unset behavior in config
checkpoint
set application/json on contents model replies
expect JSON reply to save
make savewidget css a little more generic

- use classes instead of IDs
- use .filename instead of #notebook_name
add save widget to text editor
add File/Rename
update edit page blocks after layout changes
add Mode menu to editor
Toggle line numbers in menu

use 'Toggle' text, instead of switching between Show/Hide
update mode on rename
use $.map for setting cm options