Bug 19300: Replace C4::Reserves::OPACItemHoldsAllowed

Bug 19300: Replace C4::Reserves::OPACItemHoldsAllowed

This patchset move The OPACItemHoldsAllowed logic
(issuingrules.opacitemholds) to a new class method of
Koha::IssuingRules: get_opacitemholds_policy

On the way, this patch will certainly fix the same problem as bug
19298 with onshelfholds.

Test plan:
Make sure the opacitemholds policy is correct when placing a hold at the
OPAC or the staff interface.

Followed test plan which worked as described
Signed-off-by: Alex Buckley <alexbuckley@catalyst.net.nz>
Signed-off-by: Marcel de Rooy <m.de.rooy@rijksmuseum.nl>
Signed-off-by: Jonathan Druart <jonathan.druart@bugs.koha-community.org>