[acsone-addons][event_mass_mailing][NEW]

Introduce and Use the mass mailing concept to event management
It's seem that in odoo, by convention(for python code), docstrings are quoted using double quotes (") and others string with single quote (').  Whatever your choice, try to stay consistent. ex: 
```python
confirm_mass_mailing_id = fields.Many2one(
        'mail.mass_mailing', string='Mass Mailing Confirmation',
        select=True, track_visibility='onchange')
confirmation_date = fields.Datetime(string="Confirmation Date")
```

```python
    _inherit = "event.registration"
    _mail_mass_mailing = 'Event Registration'
```
I know, it's a details. I am not, myself, always consistent. .. :smile: