Replace document repository

These commits move from storing document editions as standalone records to storing them embedded in a document record.

This will allow us to do more sane queries against the data, which will make providing features like pagination so we can handle 10,000 imported documents much simpler.

## Tasks

- [x] Write a migration to copy data from the old collection into the new collection
- [x] Remove the unused repository and model
- [ ] Tests for the new repository and model