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: Julian Maurice <julian.maurice@biblibre.com>
(cherry picked from commit c20496aea938e1faaa53daff5e2cf3d697b0eac9)
Signed-off-by: Frédéric Demians <f.demians@tamil.fr>