LP#1671596 webstaff: add Adjust to Zero option to Bill UI

LP#1671596 webstaff: add Adjust to Zero option to Bill UI

To test
-------
1. Apply patch.
2. Go to a patron record that has one or more active billings.
3. Note that in the billing grid, there is now an "Adjust to Zero"
   action.
4. Select one or more billings and apply the action.
5. Verify that the billings are adjusted to zero and that
   the adjustments show up as "payments".

Signed-off-by: Jason Etheridge <jason@equinoxinitiative.org>
Signed-off-by: Andrea Neiman <abneiman@equinoxinitiative.org>
Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>