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

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