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.
@azangru is correct build_stubbed creates a mock object that gives the illusion of being persisted ie an id etc and as a result
```
build_stubbed(:sponsorship).new_record? == false
```
where 
```
build(:sponsorship).new_record? == true
```
as the validation is on update, unless you add an end_date to the factory, then build_stubbed will fail here but build will pass