feat: Stock string for joining a channel (#7480)

Add a stock string `%1$s invited you to join this channel.\n\nWaiting
for the device of %2$s to reply…`, which is shown when a user starts to
join a channel.

I did _not_ add an equivalent to `%1$s replied, waiting for being added
to the group…`, which is shown when vg-auth-required was received. I
don't think that this would add any information that's interesting to
the user, other than 'something is happening, hang on'. And the more
text on the screen, the less likely that anyone reads it. But if others
think differently, we can also add it.

With this PR, joining a channel looks like this:

```
Msg#2003: info (Contact#Contact#Info): Messages are end-to-end encrypted. [NOTICED][INFO]
Msg#2004: info (Contact#Contact#Info): Alice invited you to join this channel.

Waiting for the device of Alice to reply… [NOTICED][INFO]
Msg#2007🔒:  (Contact#Contact#2001): You joined the channel. [FRESH][INFO]
```

src/securejoin/bob.rs

@@ -144,10 +144,9 @@ pub(super) async fn start_protocol(context: &Context, invite: QrInvite) -> Resul
             }
 
             // If we were not in the broadcast channel before, show a 'please wait' info message.
-            // Since we don't have any specific stock string for this,
-            // use the generic `Establishing connection, please wait…`
             if !is_contact_in_chat(context, joining_chat_id, ContactId::SELF).await? {
-                let msg = stock_str::securejoin_wait(context).await;
+                let msg =
+                    stock_str::secure_join_broadcast_started(context, invite.contact_id()).await;
                 chat::add_info_msg(context, joining_chat_id, &msg).await?;
             }
             Ok(joining_chat_id)