Show orphans sponsorship status

https://osraav.atlassian.net/browse/OSRA-177

- add fields for sponsorship status & current sponsor (if present) to orphan show view
- add disabled sponsorship status field to orphan edit view
- create 2 new sponsorship-related methods in the Orphan model
- make step definition for 'Unable to change field' more versatile at the cost of complexity & brittleness - worth it?

OSRA-177 #Code-Review
I think the view looks neater with the `if`: if the status is anything but 'Sponsored', only the status is shown. If the status is 'Sponsored', then the field for the sponsor is shown too. An occasional `if` in views is a necessary compromise, IMO.

The method `current_sponsor` actually returns a Sponsor object. I suspect that ActiveAdmin could handle either return - an object or a string - but I wouldn't want to implement a method with different return types.

The rationale for implementing the new methods goes beyond Demeter. `orphan.sponsorships.all_active.first.sponsor.name` is no doubt an ugly violation. But I also imagine future utility for these methods, and they abstract the implementation and allow for a cleaner `orphan.current_sponsorship` instead of `orphan.sponsorships.all_active.first`.