Match agents on import

Delivers: https://www.pivotaltracker.com/story/show/64190600

I am not at all sure about how well this work. It attempts to read all the "content holding" properties of an agent and all its subrecords, and ensure it is unique. There are probably a lot of bad edge cases. It ignores the authority id because there is already a constraint on that field and it would presumably be nugatory here. It hashes the ids of enums rather than the values. 

Note: The migration number clashes with https://github.com/hudmol/archivesspace/pulls so one of them will need to change.