The PostRepository type was using `any` (an anti pattern) rather than
`PostCollection`, and we had optional properties, which are not really
optional. This cleans up the types and updates the tests alongside them.
refs https://github.com/TryGhost/Arch/issues/41
- When an new collection is created the relational "tags" filter is now picked up properly and appropriate posts matching the tag filter are assigned and stored in the collection. Example collection filter that is now supported: `tags:['bacon']`
- Additionally cleaned up returned collection post DTOs, so we return as little data as possible and add only the fields that are needed
refs https://github.com/TryGhost/Team/issues/3170
- This implementation allows to create an automatic collection with a filter defining automatically populated posts that belong to a collection
- To populate collection using a filter the API client can send a `filter` property along with a collection request
- Filter values are compatible with the filters used in Content API (https://ghost.org/docs/content-api/#filter)
