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.
While the only possible sponsorship updates are currently inactivations, this still feels like very tight coupling. I'd prefer to customize the validator further to validate the presence of `end_date` when status changes from active to inactive