Subscriber types

Represents the state of a subscriber and what emails they should or should not be receiving. This type is meant to be fully expressive so as to consolidate the logic of determining what emails a subscriber should receive into a single place.

Note that the only types that can be set programmatically are REGULAR, UNSUBSCRIBED, GIFTED and UNPAID. All other types are meant to be user-visible and cannot be set programmatically.