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.
No problem with `build` vs. `build_stubbed`, but I am baffled by the fact that the validations are run as if on update. An object can simply be valid or invalid without being updated, one would think. This makes me even more convinced that the validator needs to be rewritten to fail only if `end_date` is missing when `active` changes to `false` and not on any update.