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.
Because the cucumber tests were failing since no end_date was provided for inactivation of a sponsorship, and the validation checked for 

`if self.end_date <= self.start_date`