fix: send Autocrypt header in MDNs

[link2xt]

Otherwise MDN signature is not checked and MDN is assigned to an email-contact.

Fixes #6947

[iequidoo]

The comment in render_mdn() tells: currently, we do not send MDNs encrypted:. Is this still true? Config::ProtectAutocrypt wouldn't work for MDNs if so.

Also, we could add some fingerprint header instead to MDNs to make them more lightweight, but adding "Autocrypt" is also fine.

[link2xt]

The comment in render_mdn() tells: currently, we do not send MDNs encrypted:. Is this still true? Config::ProtectAutocrypt wouldn't work for MDNs if so.

We send MDNs encrypted. I removed this outdated comment.

Also, we could add some fingerprint header instead to MDNs to make them more lightweight, but adding "Autocrypt" is also fine.

OpenPGP already has a packet for this. I opened an issue for using it: #6982
Adding Autocrypt header was just the easiest way to solve the issue.