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

(Paired with @jamiecobbett on this.)
* it makes it easier to see that the the validator detects disallowed images (when reading class that I've not seen before, inner classes tend to jump out immediately)
* it makes `NoDangerousHTMLInTextFieldsValidator` simpler, because it would be composed of smaller parts
* it would make it clear that `ASSETS_DOMAIN` is only used for that one purpose