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.
If I remember our thinking correctly, we wanted to test that the controller does what it does, i.e. calls the correct method on the model, passes correct instance variables and correct flash messages to the view and renders the correct view. I am not entirely sure I understand which of these tests you suggest removing and why.