Refactored mass_mailing logic to mass_mailing_somconnexio
CHANGELOG:
### CHANGED
- [#1250](https://git.coopdevs.org/coopdevs/som-connexio/odoo/odoo-somconnexio/-/merge_requests/1250) Refactored mass_mailing logic to mass_mailing_somconnexio
Related card: https://trello.com/c/9V1pgsEC/1916-massmailingsomconnexio
Edited by César López Ramírez