Allow external or query string redirect targets

For `exact` routes that are of type `redirect` we want to support targets that:

- are query strings; or
- are external URLs

This widens the validation on `exact` redirects to allow the above, and is the API side of alphagov/router#78.

I'll remove the external URL seeds in a separate PR as they need to be coordinated with a deployment and some data migrations.
I've done some experimentation, and I don't think this is much of an issue actually.  Because this is the destination we don't have the same concerns around ensuring semantic uniqueness (as well as string uniqueness) that we have the the source path.