tweaks to PO results grid

tweaks to PO results grid

Primary change is making prepayment_required not be sortable,
as it's a Boolean and Pg doesn't default a min(bool) aggregate.

Along the way:

- explicitly list all columns that should display be default
- unhide some columns that were hidden by default for some reason
  in a previous patch

Signed-off-by: Galen Charlton <gmc@equinoxinitiative.org>