[IMP] mail: make followers API explicit

Purpose of this commit is to always call followers API with explicit parameters
about existing followers management (check_existing and existing_policy
parameters). It makes code easier to read and maintain.

LINKS

Task 1933771
Task 2078313

Co-Authored-By: Thibault Delavallée <tde@odoo.com>
Co-Authored-By: Rémy Voet <ryv@odoo.com>