Attach models to the app.locals object
We need to access models from certain backend components and the current way is to require the model from pubsweet-backend. This means that you need to introduce pubsweet-backend as a dependency to the backend component, and this feels potentially troublesome to me (in the npm linking/nested dependencies/mismatched versions kind of way). So it would be nice if we just provide the models in app.locals, that express.js supports: https://expressjs.com/en/api.html#app.locals
Specifically, this is needed in pubsweet-server#15.