Validate orphan inactivation

https://osraav.atlassian.net/browse/OSRA-185

If an orphan is in an active sponsorship, his/her `orphan_status` cannot be changed from 'Active'.

I've experimented with extracting this validation from Sponsor & Orphan into a stand-alone validator. The code is here: https://github.com/NikitaAvvakumov/osra/blob/extract_inactivation_validator/lib/validators/can_be_inactivated_validator.rb. It is crude and could be refactored for clarity but I do not like it conceptually:
- leaving `if: :being_inactivated?` in the models extracts very little code into the validator, but putting the conditional into the validator makes it more awkward than it is in the models
- since the validation hinges on whether or not there are active sponsorships, the validator has to reach into the model, which feels like inappropriate intimacy
- testing is awkward due to the fact that this deals with AR associations and a simple tester class isn't sufficient. I actually ended up using real Orphan & Sponsor objects in the spec (https://github.com/NikitaAvvakumov/osra/blob/extract_inactivation_validator/spec/lib/validators/can_be_inactivated_validator_spec.rb) and the validator tests are now entirely redundant with the model inactivation tests.