Integrate with url-arbiter

This changes the ownership check to use url-arbiter instead of a local check.

This is feature-flagged (url-arbiter isn't in production yet), and when disabled the original check still applies.
`and` should generally only be used for control flow and not conditionals. See http://devblog.avdi.org/2010/08/02/using-and-and-or-in-ruby/