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
The alternative would be to build up the orphan list via `sponsor.sponsorships` rather than bypassing straight to `sponsor.orphans`. (In fact, `sponsor has_many :orphans, through: :sponsorships` becomes unnecessary.) The Sponsorship model would rely on delegation to access `sponsorship.orphan_attribute` to avoid Law of Demeter violations. This way, each table row will have easy access to `sponsorship.id`, so the route would be `admin/sponsorships/:id`.

Just thinking "out loud". Is this what you had in mind?