Custom datagrid listview.html templates can now override a

datagrid_empty block to display custom content when the datagrid is

empty. This is useful for displaying a message instead of an empty

datagrid, possibly with instructions on creating something.
The cell is wrapped in a row and cell with classes that can be styled

specially. These won't show the hover effect, like other datagrid rows.
By default, nothing is shown, for backwards-compatibility.

Made use of this in a change for Review Board. Saw my message when the

datagrid was empty, but not when populated.