4144 Add blank option to Is this a translation of another post?

https://code.google.com/p/otwarchive/issues/detail?id=4144

For admin posts, if you accidentally selected a post in "Is this a translation of another post?", you couldn't remove it. This was bad because it meant news posts could disappear from the front page. Now there's a blank option so you can undo your whoopsie.