Other: Handle Seen/Flagged IMAP flags when APPENDing a message

When an IMAP client appends a message to a mailbox, it can specify
which flags it wants the appended message to have. We need to handle
these in a proton-specific way; not-seen messages need to be imported
with the Unread bool set to true, and flagged messages need to
additionally be imported with the Starred label.

pkg/message/build.go

@@ -234,7 +234,7 @@ func buildPGPRFC822(kr *crypto.KeyRing, msg liteapi.Message, opts JobOptions) ([
 
 	hdr := getMessageHeader(msg, opts)
 
-	sigs, err := msg.ExtractSignatures(kr)
+	sigs, err := liteapi.ExtractSignatures(kr, msg.Body)
 	if err != nil {
 		log.WithError(err).WithField("id", msg.ID).Warn("Extract signature failed")
 	}