[ADD] new module "account_refund_partner"

This module add new smart buttons in the Partner view to Customer Refunds and Supplier Refunds.
You don't need to put `+` sign. Consecutive strings are concatenated automatically, but it's better to use this indentation (also PEP8 compliant) in these cases:
```
supplier_refund_count = fields.Integer(
    compute='_refund_count', string="# Supplier Refund", 
    groups='account.group_account_invoice')
```