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.
How about removing the instance variables completely?  Examples
`if Sponsorship.find(params[:id]).inactivate params[:end_date]`
`redirect_to admin_sponsor_path(sponsor_id: params[:sponsor_id])`