Improve MDN (Read receipt) message to avoid blocking from DCC servers

MDN messages (read receipt) sent by DeltaChat has always the same body content and Subject field that assumed by DCC servers (Distributed Checksum Clearinghouses that works practically on any decent email server) as bulk message and increase Fuz2 score signaling to spam filters to increase spam score.

Expected behavior

Instead of string in MDN message:
This is a read receipt for the message “Encrypted message”.
Use either some random hash or hash of body in place of “Encrypted message”.

Actual behavior

MDN messages are always the same.

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