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/receive_imf/tests.rs

@@ -3110,7 +3110,8 @@ async fn test_thunderbird_autocrypt() -> Result<()> {
     let t = TestContext::new_bob().await;
 
     let raw = include_bytes!("../../test-data/message/thunderbird_with_autocrypt.eml");
-    receive_imf(&t, raw, false).await?;
+    let received_msg = receive_imf(&t, raw, false).await?.unwrap();
+    assert!(received_msg.from_is_signed);
 
     let peerstate = Peerstate::from_addr(&t, "alice@example.org")
         .await?
@@ -3191,7 +3192,8 @@ async fn test_thunderbird_unsigned() -> Result<()> {
 
     // Alice receives an unsigned message from Bob.
     let raw = include_bytes!("../../test-data/message/thunderbird_encrypted_unsigned.eml");
-    receive_imf(&alice, raw, false).await?;
+    let received_msg = receive_imf(&alice, raw, false).await?.unwrap();
+    assert!(!received_msg.from_is_signed);
 
     let msg = alice.get_last_msg().await;
     assert!(!msg.get_showpadlock());
@@ -3200,6 +3202,27 @@ async fn test_thunderbird_unsigned() -> Result<()> {
     Ok(())
 }
 
+/// Bob receives an encrypted unsigned message with only an unencrypted Subject.
+///
+/// Test that the message is displayed without any errors,
+/// but also without a padlock, but with the Subject.
+#[tokio::test(flavor = "multi_thread", worker_threads = 2)]
+async fn test_thunderbird_unsigned_with_unencrypted_subject() -> Result<()> {
+    let bob = TestContext::new_bob().await;
+
+    let raw = include_bytes!(
+        "../../test-data/message/thunderbird_encrypted_unsigned_with_unencrypted_subject.eml"
+    );
+    receive_imf(&bob, raw, false).await?;
+
+    let msg = bob.get_last_msg().await;
+    assert!(!msg.get_showpadlock());
+    assert!(msg.error().is_none());
+    assert_eq!(msg.get_subject(), "Hello!");
+
+    Ok(())
+}
+
 #[tokio::test(flavor = "multi_thread", worker_threads = 2)]
 async fn test_mua_user_adds_member() -> Result<()> {
     let t = TestContext::new_alice().await;