new-chatroom-dialog: unsensitive the join button when account is disconnected
new-chatroom-dialog: unsensitive the join button when account is disconnected

Trying to join a room with a not connected account leads to crash (#603393).

GNOME/telepathy-account-widgets,Distrotech/telepathy-account-widgets,GNOME/telepathy-account-widgets,Distrotech/telepathy-account-widgets,GNOME/telepathy-account-widgets