disableUnreadLabelButton

Disables the unread-label button.

Suppresses the button on the current label and on any label produced by future recomputations of observeUnreadLabelState, so the dismissal survives subsequent read-state changes (for example, the auto-mark-read that follows scrollToFirstUnreadMessage). The suppression is reset when the controller is recreated, i.e. when the user leaves and re-enters the channel.