feat: Stock string for joining a channel

[Hocuri]

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]

[Simon-Laux]

LGTM

[r10s]

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'

agree. that came a bit from the time where things were much slower.

if things are not needed for channels at the end, maybe we can remove that part also for the group join