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/stock_str.rs

@@ -424,6 +424,11 @@ pub enum StockMessage {
         fallback = "⚠️ Your email provider %1$s requires end-to-end encryption which is not setup yet."
     ))]
     InvalidUnencryptedMail = 174,
+
+    #[strum(props(
+        fallback = "⚠️ It seems you are using Delta Chat on multiple devices that cannot decrypt each other's outgoing messages. To fix this, on the older device use \"Settings / Add Second Device\" and follow the instructions."
+    ))]
+    CantDecryptOutgoingMsgs = 175,
 }
 
 impl StockMessage {
@@ -750,6 +755,11 @@ pub(crate) async fn cant_decrypt_msg_body(context: &Context) -> String {
     translated(context, StockMessage::CantDecryptMsgBody).await
 }
 
+/// Stock string:`Got outgoing message(s) encrypted for another setup...`.
+pub(crate) async fn cant_decrypt_outgoing_msgs(context: &Context) -> String {
+    translated(context, StockMessage::CantDecryptOutgoingMsgs).await
+}
+
 /// Stock string: `Fingerprints`.
 pub(crate) async fn finger_prints(context: &Context) -> String {
     translated(context, StockMessage::FingerPrints).await