Enable `MappingsController#find_global` to find a site without a scheme

- Now `foo.com/bar` works, not just `http[s]://foo.com/bar`, in the "Go
  to Mapping" dialog and other places where
  `MappingsController#find_global` is called.
We don't care whether it's `http` or `https,` it just needs a scheme for `Addressable::URI.parse`. The scheme is discarded later on.