refs https://github.com/TryGhost/Team/issues/3423
- This is a convenience method that should allow fetching collections by their slug. It is a great developer experience for in case when fetching built-in collections like "featured" and "index", so can avoid an extra call to find the collection and it's ide one wants to use.
This is the first pass at an abstract class for our in-memory repository
implementations. It's based off of the existing implementations and
should speed up local development and deduplicate code
