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 have to say that the idea of rolling these two `end_date` validations into one custom `validate end_date_upon_inactivation` remains very appealing to me. It would remove the multiple conditions attached to both validations and increase the level of abstraction and readability.

Since I appear to be in the minority, however, I won't insist.