Raise an error if a path is given for a redirect_url
Raise an error if a path is given for a redirect_url

If we end up with a path for an asset's `redirect_url`, then Rails
fills in its hostname, and so redirects to `assets-origin....`, which
is only accessible on the VPN.

Rather than implement a work-around in the asset controller, instead
just require that a full URL be given.

alphagov/asset-manager,alphagov/asset-manager,alphagov/asset-manager