Add $user->hasCompany($customer->company_id) check to view, update,
delete, restore, and forceDelete methods in CustomerPolicy, matching
the pattern used by all other policies (InvoicePolicy, PaymentPolicy,
EstimatePolicy, etc.).
Without this check, a user in Company A with view-customer ability
could access customers belonging to Company B by providing the target
customer's ID.
Add cross-company authorization tests to verify the fix.
Closes#565
1ab5228347