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.
Given this is now using a separate validation method, I think it would be good to expand these specs to cover all the relevant cases from https://github.com/alphagov/router-api/blob/master/spec/models/route_spec.rb#L28-L74.  I'm finding it hard to tell from reading the code whether it covers the same cases.