Move any hard coded references to fields in API routes into the models

[ml-evs]

As part of the API refactor (before writing a new version of the API altogether), we should abstract away some of the nasty hardcoded nature of our existing routes. For example, the fields returned when summarising a given item should be annotations on the data model that can be explicitly validated (e.g. a model config that includes a summary_fields attribute). This is crucial for extending the current API to custom data models, and will be the core of the next iteration of the API.