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.
> I am baffled by the fact that the validations are run as if on update

Me too. Very much so. But rspec failed when testing the validity of the object built with `build_stubbed`. That's the only reasonable explanation we came up with.