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.
A very peculiar thing, actually. Apparently (I still can't quite believe it), `build_stubbed` **creates** a factory (i.e. gives it an id and saves it), and the validation then treates this factory as if it were being updated, not created.

And, since the factory doesn't have an `end date`, it will fail the new validation (which, you see, is triggered only on update, app/models/sponsorship.rb, L15):

`validates :end_date, presence: {scope: true, message: 'is invalid' }, on: :update`

`build`, on the other hand, does not save the factory to the database, so it will not be validated for end_date.