Store: Add a setting for the new order notification.

[justinshreve]

This PR adds a new setting for Store on .com sites, that allow them to disable the 'New order' notification.

See #16181.

To Test:

• Apply the patch from D7802-code and follow the testing steps there.
• Apply this branch, and go to your notification settings.
• Select a normal WordPress.com blog. Make sure that the 'New order' option doesn't display.
• Select a normal Jetpack blog if you have one, Make sure that the 'New order' option doesn't display.
• Select one of your AT Stores. Verify that you see a 'New order' option.
• Toggle the option, and place a new order on your test site from D7802-code. Make sure you do not receive a notification.

[matticbot]

Test live: https://calypso.live/?branch=add/store-order-notification-toggle

[justinshreve]

One thing that this doesn't do is offer an option to disable the WooCommerce email. I don't consider that a blocker for this notification though. Hooking that up here is going to require a bit more thought and work since the notification settings are stored on WordPress.com, and emails are built and sent from the end AT site.

[timmyc]

This code is looking good to me here. Verified the setting only shows when it should, and updates/persists as expected.

Per our DM, I configured my JETPACK__API_BASE and updated my sandbox with the latest diff, but did not receive order notifications as expected. I'll keep testing it out and review the phabricator diff next, but this is looking good to go here.