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
What if I stick `data: { confirm: 'Are you sure?' }` in there? That ought to provide adequate protection against crawlers. I'm not married to the current implementation, but it's so wide-spread (including the ultimate authority that is Rails Guides) that I have a hard time seeing it as fundamentally flawed.