support migrated project subject locations

migrated projects will have subject locations as per { "standard" => "S3 public url" }.
avoid n+1 subject version queries

https://github.com/airblade/paper_trail/issues/222
extract helper class
flag migrated subjects

Add support for legacy subject existing s3 locations.
pivot on subject metadata
fix schema merge conflict
Merge pull request #313 from camallen/serialise_old_subject_locations

support migrated project subject locations