Fixed #704 : forking a notebook on ctrl+enter

Hey @amol-jore, @sujitbehera27, now I've seen this "incantation" of four magic numbers a few times, so it's time to wrap it in a function in ui_utils.js.  
But what does it do?  Was it copied and pasted from somewhere?  According to this chart 
http://www.cambiaresearch.com/articles/15/javascript-char-codes-key-codes
it includes both F4 and pause/break, which seem surprising as enter keys to me.