Fix missing Objection Id columns

It was spotted whilst working on [Update "create bill run" endpoint to create a bill run](#56) that we had an issue with our [Objection models](https://vincit.github.io/objection.js/).

The previous team didn't use the convention of just calling the unique identifier in each table `id`. Instead they incorporate the table name, for example, `billing_batch_id`. **Objection** is expecting the identifier column to be `id` and is failing when in some cases.

Because of this we need to go back and specify [idColumn](https://vincit.github.io/objection.js/api/model/static-properties.html#static-idcolumn) in all our models.