7.0 account invoice unique customer ref

This PR add 2 modules: 
1/ unique_supplier_invoice_number which set a constraint on (partner_id, supplier_invoice_number) to be unique
2/ account_invoice_unique_customer_ref which set a constraint on (partner_id, name) to be unique, only for out_invoice or out_refund type account_invoice.
do here what you did [here](https://github.com/OCA/account-invoicing/pull/2/files#diff-ea02e43001b9562a6ca6660e63620aaeR66)