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.
`build_stubbed` produces an object that _looks_ persisted, that is true. The fact that validations appear to be run as if it's being updated seems very wrong though. The object should simply respond to `valid?` without triggering `on: :update`. I've just run through all 3 relevant FactoryGirl methods - `build`, `build_stubbed` & `create` - in console, and they all returned valid objects. Let's poke at this some more