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.
It's considered bad practice to modify the parameters object. We can also use the `Transition::PathOrURL` code to detect whether it starts with a scheme. Based on that, how about:
```ruby
  def find_global
    # This allows finding a mapping without knowing the site first.
    url = params[:url]
    render_error(400) and return unless url.present?

    if !Transition::PathOrURL.starts_with_http_scheme?(url)
      url = 'http://' + params[:url] # Add a dummy scheme
    end

    begin
      url = Addressable::URI.parse(url)
```