Bug 15962: Block the currency deletion if used

Bug 15962: Block the currency deletion if used

A currency should not be deleted if used by a vendor or a basket.

Test plan:
1/ Add a new currency
2/ Create a vendor using this currency
3/ Create a basket using this currency
4/ Try to delete the currency
5/ Delete the basket
6/ Try to delete the currency
7/ Delete the vendor
8/ Delete the currency

Signed-off-by: Owen Leonard <oleonard@myacpl.org>
Signed-off-by: Katrin Fischer <katrin.fischer.83@web.de>
Signed-off-by: Brendan Gallagher brendan@bywatersolutions.com