Add sponsees number limitation
CHANGELOG:
### ADDED
- [#784](https://gitlab.com/coopdevs/odoo-somconnexio/-/merge_requests/784) Add sponsees number limitation
Related card: https://trello.com/c/2MSkwhAs/1234-definir-var-de-configuraci%C3%B3n-n%C3%BAmero-apadrinados
Edited by Pelayo García