Add resource string for no conferencing account available message

resources/languages/resources.properties

@@ -356,6 +356,7 @@ service.gui.NO_CONTACTS_FOUND=No matching contacts found. Press Ctrl+Enter to ca
 service.gui.NO_CONTACTS_FOUND_SHORT=No matching contacts found.
 service.gui.NO_MESSAGE=No message
 service.gui.NO_GROUP_CHAT_ACCOUNT_AVAILABLE=No accounts, supporting multi user chat found. Check jitsi.org for more information on which protocols support multi user chat.
+service.gui.NO_ONLINE_CONFERENCING_ACCOUNT=No available conferencing account. Check https://jitsi.org for more information on which protocols support conferencing.
 service.gui.NO_ONLINE_TELEPHONY_ACCOUNT=At least one online telephony account is needed in order to make a call. Please login to one of your telephony accounts and try again.
 service.gui.NOT_AUTHORIZED=Not authorized
 service.gui.NON_EMPTY_CHAT_WINDOW_CLOSE=You're trying to close a chat with a non-sent message. Are you sure you want to close this chat?