Email Notifications: Add global settings to disable/enable admin emails

On WordPress.com, there are these two settings which allow you to disable the site-owner email notifications for new subscriptions:

These are not present on a JP-connected site though, so it's not possible to disable these emails at the site level. You can only disable the emails from the calypso side while logged into the main JP user's account.

At the moment, you can manually run `update_option('social_notifications_subscribe', 'off');` on your self-hosted site and it'll sync over and prevent the emails from being sent. But as far as I can tell this isn't made possible anywhere by default for self-hosted sites.

Start by locating the handling of the `social_notifications_subscribe` option and the existing site-level notification settings. The change is complete when self-hosted, JP-connected sites can enable or disable the site-owner subscription emails through global settings and the selected setting prevents or permits those emails.

Written by the indexing model from the issue text.