feat: Don't affect MimeMessage with "From" and secured headers from encrypted unsigned messages

If a message is encrypted, but unsigned:
- Don't set `MimeMessage::from_is_signed`.
- Remove "secure-join-fingerprint" and "chat-verified" headers from `MimeMessage`.
- Minor: Preserve "Subject" from the unencrypted top level if there's no "Subject" in the encrypted
  part, this message is displayed w/o a padlock anyway.

Apparently it didn't lead to any vulnerabilities because there are checks for
`MimeMessage::signatures.is_empty()` in all necessary places, but still the code looked dangerous,
especially because `from_is_singed` var name didn't correspond to its actual value (it was rather
`from_is_encrypted_maybe_signed`).

src/peerstate.rs

@@ -705,9 +705,7 @@ pub(crate) async fn maybe_do_aeap_transition(
                 // addresses with an MUA.
                 && mime_parser.has_chat_version()
                 // Check if the message is signed correctly.
-                // If it's not signed correctly, the whole autocrypt header will be mostly
-                // ignored anyway and the message shown as not encrypted, so we don't
-                // have to handle this case.
+                // Although checking `from_is_signed` below is sufficient, let's play it safe.
                 && !mime_parser.signatures.is_empty()
                 // Check if the From: address was also in the signed part of the email.
                 // Without this check, an attacker could replay a message from Alice