[Bug 946267] Showfor: be less clever in the admin.

Trying to stop duplicate default versions kind of broke the UX of the admin, and having multiple defaults for one product doesn't blow up. Not even a little bit.

The only down side is that if two defaults are selected, one of them will be arbitrary chosen to show to users we don't use UA detection for. So Chrome users, or IE users, or iPhone users. Chrome chooses the last default option, which has the lowest version number. I didn't test any other cases. Hopefully it doesn't end up mattering.

Additionally, apparently you can't have '.' in SlugFields. You can insert dots into them via migrations though. So that's weird.

r?