Sponsorship functionality & UI

PT story https://www.pivotaltracker.com/story/show/78039462

- Add functionality to link sponsor & orphan (i.e. create sponsorship)
- Sponsored orphans shown on sponsor's page
- Sponsorships can be ended by clicking link next to orphan's name on sponsor page
- Establishing a sponsorship marks orphan as 'Sponsored'
- Only Active Unsponsored orphans displayed when creating new sponsorship link
- Only one sponsorship relationship can be established between a sponsor and orphan (enforced in db)

UI is not final as client has sent new specifications
There is no other way to do 'delete' to the best of my knowledge. Most if not all browsers cannot send DELETE requests natively, so here Rails fakes it with JavaScript. The only other alternative would be to use a form and a POST request.

Since the route is defined as `delete route, to: controller, as: path` (or `match ... via: :delete`), issuing a simple GET request to this URL won't destroy the resource, if that's a concern ([ref](http://guides.rubyonrails.org/getting_started.html#deleting-articles)).