Grey out and disable message send button on conversation page when the other user is offline

I note that offline message delivery does not seem to work yet in Jami. I remember a discussion in the fediverse with a Jami developer who said this feature is not yet implemented. But the UI does nothing to indicate this. So unless users know better, they will continue trying to send messages to other users when that are offline, not understand why it's not working, and get increasingly frustrated. The solution I propose is a fairly simple tweak to the UI, where the message send button is greyed out and disabled when the other user in a conversation is offline.

It would be even better if there was a message above the message typing bar saying something like: "text messages cannot be delivered to other users while they are offline"