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 not comfortable with these verbose descriptions.  Is there any way to make a `context '#inactivate'` and nest these two inside, with something like `example 'when previously sponsored'`