Limit sponsorships to active sponsors

JIRA story https://osraav.atlassian.net/browse/OSRA-55

- create Sponsor#eligible_for_sponsorship method and use it to limit display of 'Link to Orphan' button
- create Status#active method
- attendant specs & scenarios

Created https://osraav.atlassian.net/browse/OSRA-153: implement `before_create` validations in the Sponsorship model to ensure that sponsor and orphan are eligible for a new sponsorship
Can't do `describe` here, but can do `specify` which, I agree, would read better