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.
Again, if I remember our thinking correctly, the #incativate method of the model returns something because we are checking for this in the controller action (app/admin/sponsorship.rb, lines 13-16): 

+ if @sponsorship.inactivate params[:end_date]
+   flash[:success] = 'Sponsorship link was successfully terminated'
+ else
+   flash[:warning]= @sponsorship.errors.full_messages || 'Sponsorship not terminated'