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 aware this is an artifact you simply moved, but unless this controller is sending these to a view for use in rendering, it should matter not how this method does what it does, only the end result based on input.  So, I'm recommending purging this test