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.
We were working on two independent branches, both created from `develop`, which gave us all kinds of headache, so we might have dulicated a spec. But I don't know how to compare two PRs in GitHub to see what we have re-written.