When starting an onboarding/offboarding plan on an employee, activities
were only created for the users that were part of the HR Officer group.
Regular users can't access the `hr.employee` model and it's not possible
to have activities on `hr.employee.public`.
A new model has been created to manage those activities and is only
accessible to the user and their manager.
task-3151758
closesodoo/odoo#111505
Signed-off-by: Kevin Baptiste <kba@odoo.com>