Fixed some embarrassing errors in jk_across_pages

- The destroy() function redirected the user to tumblr.com/undefined, preventing uninstallation
- The extension tried to work when editing a post

Other fixes:
- Improved, renamed, and removed "Beta" label from the "View entire post" functionality
- Removed the separator in the Preferences
- Wrapped the destroy function in a try-catch to avoid other problems from preventing an uninstall. (That wouldn't have fixed that one, but it's worth having.)
- Used jQuery's event.which instead of duplicating its functionality

This had a few pretty bad bugs, so the sooner you can merge it in the better. Sorry about that.