Add Change Tariff BA button
CHANGELOG:
### ADDED
- [#772](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/772) Add Change Tariff BA button
Related card: https://trello.com/c/tovQbhNh/1173-canvi-tarifa-contractes-ba-a-tarifa-sense-fix
Edited by Pelayo García