5370: Show that all external images have been downloaded for an email

👍 Gathering votes

It would be good to have a way of telling whether "download external images" has been applied to an email or not. At present the only way of being sure that an HTML email is future-proof would be to transfer it to an archive folder called something like “Baked-in”. A flag to indicate whether or not there are undownloaded remote images would round out the feature.