[ADD] Add a new graph view in mailing list contacts to track list subscr...

...iptions and unsubscriptions + custom filters provided

- Task: https://www.odoo.com/web?debug=#id=9618&view_type=form&model=project.task
- Pad: https://pad.odoo.com/p/r.Lgn9WTUeZrpgZeSZ
You could be less verbose for this method, for example:
return {'value': {'unsubscription_date': opt_out and fieds.datetime.now() or False}}