Reject images that are not on the assets domain or a relative path

(Paired with @jamiecobbett on this.)
I don't think we need to give an example of another domain. We should also be consistent with the language - domain versus website. What about:

> Images are only allowed if on a relative path (ie hosted on www.gov.uk) or on the GOV.UK assets domain: `assets.digital.cabinet-office.gov.uk`. Markup containing images hosted on other domains will be rejected with a `422` error code.