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.
Especially if we roll the current `end_date` validations into a single custom validator, and test it well, I would prefer to limit the acceptance test to one sad path scenario with a generic "Ending with an invalid end date" description.