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.
This refers to Brian's comment: https://github.com/AgileVentures/osra/pull/226#discussion_r22124142

We played with `example`, and I thought we changed it to `it` (but apparently we didn't). As for the amount of description, this needs to be sorted out with Brian — he is all for brevity, I understand :smile: