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

(Paired with @jamiecobbett on this.)
Having thought about how to express the behaviour in the README, I think we should allow images with an absolute path on www.gov.uk as well so that the content can link to the image on GOV.UK when they are previewing/hosting it internally. So we'd need something like:
```ruby
ALLOWED_IMAGE_HOSTS = ['www.gov.uk', 'assets.digital.cabinet-office.gov.uk']
```
and in the method:
```ruby
ALLOWED_IMAGE_HOSTS.exclude?(uri.host)
```