O294 sponsorship end date validation

https://osraav.atlassian.net/browse/OSRA-294
This is connected to:  https://osraav.atlassian.net/browse/OSRA-298

Sponsorship model validatation for end_date
- can't be before start date
- can't be blank

plus rspec and cucumber tests

Andrey and me worked on this.
@PurityControl :+1: 
After a fresh look at this, ideally sponsorship inactivation and change of orphan status to previously sponsored should be an atomic process, both or none. Future chore?