CiviEvent: Participation confirmation screen

There should be a new screen for event participation confirmation, injected into the CiviEvent workflow before the first screen.

The access to this screen should require a valid civicrm_contact.hash and would be presented to existing participants (i.e., would be skipped on initial registration).

This screen should be put before the first wizard screen and should allow the participant to:

1. Cancel their participation (for now without a refund if the event is paid).
2. Confirm their participatoin if the participant is in any of the pending states (pending from waitlist, pending after approval, etc.).

If the participant chooses to confirm, they should be able to walk the CiviEvent wizard (with the fields properly pre-populated) and edit some of the fields. They would be required to pay for the event (if the event is paid).