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.
That is actually what we discussed while re-writing failing Cucumber tests. When we were about to finish re-writing we realized that the tests were failing because the end_date was the same as the start_date, and this was not allowed by the model.

In any case, I think it would make much more sense to have a separate Cancel button that would delete sponsorships created accidentally. Much better than end them and create sponsorships that in fact never existed.