[feature] Allow to disable notifications for all organizations or keep everything disabled except notifications for specific organizations

[nemesifier]

Current buggy situation:

• superadmin A decides to receive no email notifications or no notifications at all, to do so, they must explicitly disable all notifications (which is time consuming)
• a new organization called Snakeoil is created in the system
• superadmin A gets the notification settings records created for Snakeoil, defaulting to enabled notifications
• superadmin A starts receiving notifications for Snakeoil even though they decided to turn notifications off

Possible solution:

Introduce a general setting, which allows to turn off web and/or email notifications for all organizations (eg: a record like NotificationSettings(organization=None, type=None, user=user, email=False, web=True) could indicate that email notifications are disabled but web notifications are enabled for a specific user, regardless of the organization).

When all notifications are turned off using this method, the system must flag all notification settings relating to the different orgs as disabled.
Vice versa, if all notifications are turned on, the system must flag all notification settings relating to the different orgs as enabled.

When a user disables all notifications for all orgs, it must be possible for the user to enable notifications for a particular organization they're interested in, so that all notifications are disabled except for specific notifications for a specific organization.
This would allow a user to have notifications enabled only for a particular organization while maintaining notifications disabled for all other organizations, even for organizations that would be created in the future.

This feature must be available through the REST API, that is, it shall be possible to enable/disable all notifications for all orgs via the REST API with one HTTP request.