Show message when edition diff is empty
Show message when edition diff is empty

This is related to the work on redesigning the edition show page.
Editors were confused by the blank diff page, so it now shows a
message when the diff is blank. Also tidied up copy.

robinwhittleton/whitehall,alphagov/whitehall,YOTOV-LIMITED/whitehall,askl56/whitehall,alphagov/whitehall,YOTOV-LIMITED/whitehall,hotvulcan/whitehall,askl56/whitehall,alphagov/whitehall,askl56/whitehall,YOTOV-LIMITED/whitehall,robinwhittleton/whitehall,ggoral/whitehall,YOTOV-LIMITED/whitehall,askl56/whitehall,hotvulcan/whitehall,ggoral/whitehall,alphagov/whitehall,ggoral/whitehall,hotvulcan/whitehall,robinwhittleton/whitehall,robinwhittleton/whitehall,hotvulcan/whitehall,ggoral/whitehall