LP#1729435: Allow saving Bill Full Details grids

LP#1729435: Allow saving Bill Full Details grids

For a web staff eg-grid to save and retrieve its settings, it needs a unique
persist-key. This commit adds one for the Bills grid and Payments grid on the
patron bills Full Details view (right-click a bill, choose Full Details, then
click the Details tab).

Signed-off-by: Remington Steed <rjs7@calvin.edu>
Signed-off-by: Michele Morgan <mmorgan@noblenet.org>
Signed-off-by: Jennifer Pringle <jennifer.pringle@bc.libraries.coop>
Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>