This change does two things:
1. The old `BounceRecord`, `BounceType` and `BounceSubtype` types are
   removed from the `auth_db` module into `bounces`, so that they can
   more easily be re-used when writing bounce/complaint data to Redis.
2. Those types are renamed to `DeliveryProblem`, `ProblemType` and
   `ProblemSubtype` respectively, and the `bounces` module is renamed to
   `delivery_problems`. That may seem weird at first, but I have good
   reason for it and am open to alternative names if anyone can suggest
   something better.
   Historically we've always treated complaints as a type of bounce
   event, but that's inaccurate because bounces are never preceded by a
   delivery event whereas complaints are always preceded by one. This
   has caught me out in the past when analysing metrics and expecting a
   blanket sum of `deliveries + bounces = sends` to be true. It isn't.
   It's only true if you filter complaints from bounces before
   calculating the sum.
   Because of that, I wanted some different nomenclature that didn't use
   "bounces" as an umbrella term for "bounces and complaints". It won't
   affect the names we use in the metrics but I think it's important for
   language we use in the codebase to be as precise as possible. Things
   like "delivery error" or "delivery failure" also seemed inaccurate
   for the same reason; a complaint implies that the actual delivery
   succeeded. Hence "delivery problems", which seems generic enough to
   legitimately include both event types.