feat: Add device message about outgoing undecryptable messages (#5164)

Currently when a user sets up another device by logging in, a new key is created. If a message is
sent from either device outside, it cannot be decrypted by the other device.

The message is replaced with square bracket error like this:
```
<string name="systemmsg_cannot_decrypt">This message cannot be decrypted.\n\n• It might already help to simply reply to this message and ask the sender to send the message again.\n\n• If you just re-installed Delta Chat then it is best if you re-setup Delta Chat now and choose "Add as second device" or import a backup.</string>
```
(taken from Android repo `res/values/strings.xml`)

If the message is outgoing, it does not help to "simply reply to this message". Instead, we should
add a translatable device message of a special type so UI can link to the FAQ entry about second
device. But let's limit such notifications to 1 per day. And as for the undecryptable message
itself, let it go to Trash if it can't be assigned to a chat by its references.

src/config.rs

@@ -291,6 +291,9 @@ pub enum Config {
     /// Timestamp of the last time housekeeping was run
     LastHousekeeping,
 
+    /// Timestamp of the last `CantDecryptOutgoingMsgs` notification.
+    LastCantDecryptOutgoingMsgs,
+
     /// To how many seconds to debounce scan_all_folders. Used mainly in tests, to disable debouncing completely.
     #[strum(props(default = "60"))]
     ScanAllFoldersDebounceSecs,