format images of signed or plain rich text messages

[tomholub]

Still todo: inline images of signed, or plain (rich text) messages

Originally posted by @tomholub in #2398 (comment)

These should use MIME email embedded image references like other email clients do.

[tomholub]

add test too

[tomholub]

this is a nuanced (and potentially complicated) issue - please see how inline images may be embedded in the body of an email when sending a plain rich text email.

You can see how Gmail does it - they encode it as separate parts of the mime email, and refer to it using some sort of a reference that other email clients recognize, within the html body.