Inject document type not a pre-scoped model to repositories.

In each case in the `RepositoryRegistry`, `specialist_document_editions` was
just a pre-scoped `SpecialistDocumentEdition` by document type.

I have refactored this scoping to happen within the Repository, as every place
that a repository was being used was being scoped by document type, and there
is no scenario we have encountered where this is not the case.

Further, part of the point of using the repository pattern is to concrete all
datastore access/operations inside the repository, so having this occur outside
the repository smelled a little to me.

This will provide further benefits when we introduce a Single Source of Truth
to list the available document types, making it much easier for us to use that
Single Source of Truth in future to simplify repository access.
Merge pull request #304 from alphagov/remove-scoped-editions-from-repository-registry

Inject document type not a pre-scoped model to repositories.