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.
For a validation, it is sufficient to add an error - see L69 below. The cancel button is a good idea that can wait until the client requests it. In the meantime, the specification is that `end_date` cannot _precede_ `start_date`, so unless `end_date` < `start_date` the object is valid.