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.
Same here... if the conditional is negated then true will return from the evaluation.
`unless end_date > start_date` (negated if) or
`unless end_date >= start_date`
Which of these to use should also be discussed.  If the user accidentally creates a sponsorship, should they be able to immediately end it? Hmm