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.
What about creating a custom validator: when status changes from active to inactive, it validates that `end_date` is present _and_ that `end_date` is not set to be earlier than `start_date`?