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.
@razorcd Especially if the `unless` bothers you, it can be definitely replaced here with `if end_date < start_date` since that also reduces the number of conditions from "later than or the same as" to "earlier than".