Add two new fields to registration

- New field `Registration`.`Total Due` will determine the total a user is due based upon the time they completed the registration.
  - This field is used by the client to determine how much is required at payment time.
- New field `Registration`.`Completed Timestamp` will record the moment when the registration is submitted/confirmed.
- Neither of these fields can be edited by the client.  They are always calculated/determined on the server.

Addresses #69