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
`current_sponsorship.sponsor if currently_sponsored?`