[UX] Add a site-wide "E-mail address(es) to notify" setting

This is something that has come up multiple times in https://github.com/backdrop/backdrop-issues/issues/2117, as many people believe that the 'site_mail' is a TO address, when in fact, it is meant to be a FROM address.

We do already have one TO address for notifications in core, it's part of the update module and is located at admin/reports/updates/settings as mentioned by @olafgrabienski in this comment.

It has been recommended that we move this setting someplace more global and re-purpose it for other uses, such as: * Update status notifications (also individually configurable) * default TO for the contact module (also individually configurable) * notifications for severe errors in dblog (see https://github.com/backdrop/backdrop-issues/issues/2229) * admin notification on new accounts with pending approval (see https://github.com/backdrop/backdrop-issues/issues/4428) (also maybe will be individually configurable soon?) * other things we have not yet imagined (?)