Add Callback model class

Conflicts:
	storage_service/locations/models.py
Add new File model
Fedora: record each file to the database

In the absense of persistent UUIDs originating in the storage service,
storing the checksum and source ID allows the file to be recognized
when it comes back around again.
Add post-store AIP callbacks

Conflicts:
	storage_service/locations/api/resources.py