feat: Auto-restore 1:1 chat protection after receiving old unverified message

I.e. add the "Messages are guaranteed to be end-to-end encrypted from now on." message and mark the
chat as protected again because no user action is required in this case. There are a couple of
problems though:
- If the program crashes earlier than the protection is restored, the chat remains
  protection-broken. But this problem already exists because `ChatId::set_protection()` is never
  retried.
- If multiple old unverified messages are received, protection messages added in between don't
  annihilate, so they clutter the chat.

src/receive_imf.rs

@@ -762,6 +762,7 @@ async fn add_parts(
     let state: MessageState;
     let mut hidden = false;
     let mut needs_delete_job = false;
+    let mut restore_protection = false;
 
     // if contact renaming is prevented (for mailinglists and bots),
     // we use name from From:-header as override name
@@ -1006,6 +1007,13 @@ async fn add_parts(
                             )
                             .await?;
                     }
+                    if let Some(peerstate) = &mime_parser.decryption_info.peerstate {
+                        restore_protection = new_protection != ProtectionStatus::Protected
+                            && peerstate.prefer_encrypt == EncryptPreference::Mutual
+                            // Check that the contact still has the Autocrypt key same as the
+                            // verified key, see also `Peerstate::is_using_verified_key()`.
+                            && contact.is_verified(context).await?;
+                    }
                 }
             }
         }
@@ -1741,7 +1749,16 @@ RETURNING id
         // delete it.
         needs_delete_job = true;
     }
-
+    if restore_protection {
+        chat_id
+            .set_protection(
+                context,
+                ProtectionStatus::Protected,
+                mime_parser.timestamp_rcvd,
+                Some(from_id),
+            )
+            .await?;
+    }
     Ok(ReceivedMsg {
         chat_id,
         state,