Inactivate sponsorship

PT story https://www.pivotaltracker.com/story/show/79590362

- replace deletion of sponsorship with setting `.active = false`
- modify sponsor view to show inactive sponsorships separately from current ones
- remove unnecessary routes & use ActiveAdmin `member_action` for inactivation (thanks, @sampritipanda!)
- ensure that actively sponsored orphans cannot be re-sponsored
- ensure that sponsorship inactivation correctly re-enables sponsoring of the same orphan
would # rather than . better describe instance method?