feat: put display names in encrypted headers (#5166)

[Septias]

If we encrypt the message, we strip display names from to and from fields in the unencrypted headers but put this information in the encrypted part. Otherwise, we put the display names in the unencrypted headers.

• [x] Core should overwrite the display name from the protected from field but not the whole from field, because that would allow forgery.
• [x] Core should overwrite the to field with the protected one.

close #5166

[Septias]

@link2xt are you fine to merge or do you want to look over it again?

[link2xt]

This does not currently check if the chat is verified, right?

I wonder if it is fine to do for non-verified chat since it does not work with Thunderbird, need more opinions from others.

[iequidoo]

I wonder if it is fine to do for non-verified chat since it does not work with Thunderbird, need more opinions from others.

For me it's better not to complicate the code with additional checks for compatibility with other MUAs if it's not critical. Anyway we need to create a Thunderbird issue (at least i can't find such one).

[iequidoo]

Taking this over, people ask for this feature here: https://support.delta.chat/t/technical-question-encrypt-profile-names/3049 CC @Septias