Encrypt MDNs (read receipts) #5168
Description
Currently MDNs (read receipts) are an exception in chatmail, we allow to send them unencrypted outside because Delta Chat historically does not encrypt them. One of the primary reason is that if encrypted MDN is received and cannot be decrypted it would turn into a message bubble "[ This message cannot be decrypted. ..." which is confusing when users try to debug it sitting together next to each other because on the sender device there is no message bubble and the sender will say that they did not sent anything.
Improving handling of unencrypted incoming messages is a separate topic, see:
#5162 (comment)
It anyway makes sense to prepare a PR that encrypts MDNs already and maybe even merge this. Maybe the problem of undecryptable MDNs is not that bad, undecryptable messages from unknown groups after resetup are much worse and we anyway want to solve this so this should not be a blocker for encrypted read receipts.
MDNs may still be sent unencrypted if there is no Autocrypt key for the contact and we need to ensure they have multipart/report type in this case (#5152). If MDN is sent unencrypted, we can still skip Autocrypt key as it is not useful. Autocrypt standard requires that multipart/report messages are ignored when processing Autocrypt headers to avoid triggering "reset" state by MDNs.