nbconvert --to script

Use extension and mime type from NB metadata
'Download as' script
Update docs about kernel_info_reply
Fix asyncy nbconvert to download
Basic infrastructure for new texteditor component
Loading a file works
Saving files works
Refactor editor into Editor class
Enable codemirror search/replace shortcuts
Update text editor for new contents API
Override Ctrl-S shortcut to save text editor contents
Use NotificationArea in the text editor
Set page title for editor
Styling to make it clearer where the editor is
Better way of saving through CodeMirror

This should support whatever keyboard shortcuts are configured for the
'save' command.
update payloads for msg spec 5

- remove auto_rewrite_input
- remove doctest_mode
- remove exit key from ask_exit
- rename 'edit_magic' to 'edit'
- remove screen_lines from page

remaining payloads ('edit', 'ask_exit', 'set_next_input', 'page')
are documented as deprecated in the msg spec
Add the . into file_extension
Missed a couple of dots
DEV: Add various events.
Add filename to header area