[FIX] stock: owner of quants

When an owner is defined on a quant, the current behavior is to to allow
only the owner to use it. This is restrictive for internal transfers:
when a quant is owned by Partner A, only a picking with owner Partner A
can select it.

The fix is to make things a bit less restrictive: when a quant is owned
by Partner A, a picking without owner or with Partner A as owner can
select it.

opw-687207