Fixed editor wrongly executing undo operation

Hello there,

there's a bug in the editor that triggers undo action when user presses the (right) alt+z key combination, which is used in Polish keyboard layout to achieve the letter "ż". My commit duplicates changes from the official repo of pagedown where it was fixed: https://code.google.com/p/pagedown/source/detail?r=7bf1d3cd5ed557523db65bf5615d53e465816a96#

If the pull request gets merged please update the dependency in django-pagedown, cause that's where I'd like to benefit from it :)