Add sponsorship code
CHANGELOG:
### ADDED
- [#779](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/779) Add sponsorship code
Related card: https://trello.com/c/c0kegJ1W/1233-generar-c%C3%B3digo-de-apadrinamiento
Edited by Pelayo García