Make the "Keyboard Shortcuts" dialog in Differential less hideous

Summary:
Ref T13515. Adding "\" ("Open in External Editor") made this slighlty worse, but it was already pretty bad.

Long ago the keys had a special style on them, but this got changed and dropped somewhere around D16568 -- although at the time, I think they still had a grey background (see T11654).

Some later change removed this background.

Put the background back and separate the keystrokes into groups.

Test Plan: {F7370615}

Maniphest Tasks: T13515

Differential Revision: https://secure.phabricator.com/D21141