Bug 31051: Show patron savings on the OPAC

Bug 31051: Show patron savings on the OPAC

This new feature shows a patron how much they have saved by using the
library rather than purchasing items. Savings are calculated based on
item replacement prices. The system preference allows you to choose
where to display the savings - the user page, the summary box on the
OPAC homepage, or the checkout history page.

To test:
1. Update database and restart services
2. Confirm the new OPACShowSavings system preference is found in the
OPAC tab of Administration -> global system preferences. There should be
no options selected.
3. Find a patron with a checkout history, or check out a few items to a
patron.
4. Test with different values of OPACShowSavings and confirm that
savings are shown in the expected places.

Sponsored-by: Horowhenua Libraries Trust
Signed-off-by: Marie-Luce <marie-luce.laflamme@inlibro.com>
Signed-off-by: Axelle <axelle.clarisse@univ-amu.fr>
Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Fridolin Somers <fridolin.somers@biblibre.com>
Signed-off-by: Tomas Cohen Arazi <tomascohen@theke.io>