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'm still not sure we need this.  The "return" prefix could also be removed from L42, but it's L44 that bothers me.  Cannot we invert the whole `if` and remove this?
`if self.end_date < self.start_date` == `unless end_date >= start_date`