Return schemas from services instead of models?

[joelvdavies]

Currently we mostly return models from service functions. This means that cases where the repo output could require further processing e.g. adding fields from other repos, or in the object-storage-api where we insert presigned urls, the database model would no longer be sufficient to hold the data. So I think it could be worth modifying so models are kept only for the repo and are used in the services, but the services themselves only return schemas.