Change mailing list subscription to when the user validates their email (#9384)
* Change mailing list subscription to when the user validates their email
This change also adds support for subscribing the user to an onboarding group.
Users in this group will get custom onboarding message(s),
to help them along on their usage of the platform.
* Update tests
* Update readthedocs/core/signals.py
Co-authored-by: Santos Gallegos <stsewd@proton.me>
* Migrate date NULL values to `timezone.now()`
* TODO note to migrate the signal into a Celery task
* Typo
* Darker files
* Update readthedocs/core/signals.py
Co-authored-by: Eric Holscher <25510+ericholscher@users.noreply.github.com>
Co-authored-by: Manuel Kaufmann <humitos@gmail.com>
Co-authored-by: Santos Gallegos <stsewd@proton.me>