[8.0] invoicing_voucher_killer

This module is migrated from OCA/bank-statement-reconcile 7.0.

As it's purpose is to remove the Pay buttons from the invoice forms, it is related to payment. 
This bank-payment repository is therefore a better place for it.

Since bank statements do not use vouchers in 8.0, this module is even more useful than in 7.0.

The module migrates trivally from 7.0.
Please put it in a separate file README.rst, and explain the description indicating that only users that belongs to the group "invoicing_voucher_killer.invoice_voucher_user" can see this button.