Allow admin to update email subscriptions for users

[jmiranda]

At the moment we only have a few emails being sent by the system. Eventually we'll allow this to be more customizable (email templates) and event-driven (shipment shipped, product created, order placed), but for now we just need a simple way to subscribe/unsubscribe from emails so I'm planning on making email subscription a role (SHIPMENT_NOTIFICATION_RECIPIENT, PRODUCT_NOTIFICATION_RECIPIENT). When we refactor the email notification feature we can make this a little more useful by allowing a customizable subscription mechanism (based on entity, location, and event).