Converse UI should present 'offline settings' when queue is closed

[guusdk]

When a workgroup queue is not open (eg: no logged in agents), then the Converse UI should not show a functional chat button (the button that's used to start a new chat).

That button should either be removed, or be disabled somehow.

Note that ideally, the state of the button is dynamically adjusted, to ensure that, when the queue state changes, the button on the page (that has already been loaded) also does.

[guusdk]

Confusingly, when the queue is closed, end-users clicking on the chat button appear to join a chatroom immediately. As I assume their messages will be lost, that's undesirable.

[guusdk]

Falling back to a chatroom was added as a feature, not a bug, in https://github.com/guusdk/openfire-fastpath-plugin/blob/64d9a35e693ec7429d958dc1cc6d7e03738566ae/classes/ofmeet.js#L114

We've discussed this desired functionality in #48 and agreed to replace this with a new 'offline setting' instruction "offer to join chatroom" (#51)