Send and apply MDNs to self (#7005)

We currently synchronize "seen" status of messages by setting `\Seen` flag on IMAP and then looking
for new `\Seen` flags using `CONDSTORE` IMAP extension. This approach has multiple disadvantages:
- It requires that the server supports `CONDSTORE` extension. For example Maddy does not support
  CONDSTORE yet: https://github.com/foxcpp/maddy/issues/727
- It leaks the seen status to the server without any encryption.
- It requires more than just store-and-forward queues and prevents replacing IMAP with simpler
  protocols like POP3 or UUCP or some HTTP-based API for queue polling.

A simpler approach is to send MDNs to self when `Config::BccSelf` (aka multidevice) is enabled,
regardless of whether the message requested and MDN. If MDN was requested and we have MDNs enabled,
then also send to the message sender, but MDN to self is sent regardless of whether read receipts
are actually enabled.

`sync_seen_flags()` and `CONDSTORE` check is better completely removed, maybe after one
release. `store_seen_flags_on_imap()` can be kept for unencrypted non-chat messages.

One potential problem with sending MDNs is that it may trigger ratelimits on some providers and
count as another recipient.

src/reaction.rs

@@ -747,13 +747,23 @@ Content-Disposition: reaction\n\
             alice_reaction_msg.id.get_state(&alice).await?,
             MessageState::InSeen
         );
-        // Reactions don't request MDNs.
+        // Reactions don't request MDNs, but an MDN to self is sent.
         assert_eq!(
             alice
                 .sql
                 .count("SELECT COUNT(*) FROM smtp_mdns", ())
                 .await?,
-            0
+            1
+        );
+        assert_eq!(
+            alice
+                .sql
+                .count(
+                    "SELECT COUNT(*) FROM smtp_mdns WHERE from_id=?",
+                    (ContactId::SELF,)
+                )
+                .await?,
+            1
         );
 
         // Alice reacts to own message.