Remove `ReceiptSerializer`
Add `users.activated_at` column

Use this column in addition to `users.confirmed_at` to determine if a
user is allowed to sign in.
Ensure inactive users cannot sign in
Activate users after they receive a delivery
Add `users.activated_at` to track activation

Overloading `users.confirmed_at` is problematic. Although registrations
aren't implemented, when they are this column will be used to determine
if a user has access to the system and `users.confirmed_at` will ensure
the user requesting an account owns the email address associated with
it.