Implement ranked qualification waiting list

[dunkOnIT]

The first of several event-specific waiting list PR's, see #666.. This will create backend support for event-specific waiting lists in cases where ranked qualifications are used.

Implementation Questions:

• Should we have model validations for consistency between competing_status and event_registration_state? Or is comprehensive testing enough?
• This implementation requires the Registration model to call the competition API - are we ok with that?