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.
If these two are only being used once, would it make more sense to use `@start_date - 1.month` instead of making new instance variables here prior to each test running.  Or, making it inside the relevant test