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.
After we pushed the corrected PR, this comment became hidden, while I think it may still be open for discussion. So here's the link: https://github.com/AgileVentures/osra/pull/226#discussion_r22144905