Bug 21701: Have PayPal optionally return to originating OPAC url rather than OPACBaseURL

Bug 21701: Have PayPal optionally return to originating OPAC url rather than OPACBaseURL

This enhancement will allow a library to choose whether to use the alias of the
current OPAC or the value of OPACBaseURL as the return url when making payments
via PayPal.

To test:
Note: you need to have PayPal enabled on your system (either Sandbox mode or
Live) and be using an alias URL for your OPAC.

1. Apply the patch.
2. With the option "PayPalReturnURL" set to "OPACBaseURL", log into a card via
the OPAC and start the process to pay in PayPal (you can either complete the
payment or cancel from the PayPal page). When you are returned to the OPAC, the
domain will be the value of OPACBaseURL.
3. With the option "PayPalReturnURL" set to "OPAC's alias", repeat the above
sequence to make a payment in PayPal. When you are returned to the OPAC, the
domain will be your current alias.

Signed-off-by: Brendan Gallagher <brendan@bywatersolutions.com>
Signed-off-by: Kyle M Hall <kyle@bywatersolutions.com>
Signed-off-by: Martin Renvoize <martin.renvoize@ptfs-europe.com>