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.
Merge pull request #383 from alphagov/go_to_mapping_can_work_without_scheme

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