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]
```

deltachat-rpc-client/tests/test_securejoin.py

@@ -164,7 +164,7 @@ def test_qr_securejoin_broadcast(acfactory, all_devices_online):
 
         if please_wait_info_msg:
             first_msg = chat_msgs.pop(0).get_snapshot()
-            assert first_msg.text == "Establishing connection, please wait…"
+            assert "invited you to join this channel" in first_msg.text
             assert first_msg.is_info
 
         member_added_msg = chat_msgs.pop(0).get_snapshot()