Extra binstub
Added slugs to persistence + tweaked creations to contain objects
Generate a slug in the domain object
Basic topic persistence

Put topics in the `AudienceRepository` for the moment. Not sure whether
that's the best place, or whether it make sense to split them up later.

The slugging functionality is duplicated in topics and audiences at the
moment. I want to keep it separate for the moment in case the needs are
different.