Make users bannable.

- Short-circuit non-get requests for banned users
- Close #247
Merge pull request #316 from edpaget/ban-users

Make users bannable.
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
scrub sql error messages from json_responses

do not expose any internal schema / access control messages in the response object.
keep terminology in the api layer consistent
Merge pull request #313 from camallen/serialise_old_subject_locations

support migrated project subject locations
Merge pull request #317 from camallen/scrub_relation_update_sql_responses

scrub sql error messages from json_responses