fix(heroku): remove SERVER_NAME setting
fix(etag): disable etags handling

SD-293
fix(heroku): update eve-elastic version to make use of source param
fix(auth): make auth error responses json

SD-312
fix(validation): return errors with predefined keys

eg for unique we return `{'email': {'unique': 1}}`

SD-317
fix(validation): add custom response to rest of used validators

SD-317
fix(users): make password minlength match ui
fix(cache): force cache revalidation