Add a message for the Livetable's empty state

When the Livetable doesn't have any items, we should display a message stating so.

This would be consistent with the way we handle empty states for other comments, attachments, etc. (using the .noitems class)
Also this could be a distinctive mark that the livetable loaded fully (but it's empty). This state would differentiate from the 'still loading' state or a 'fail' state.