Initial work on the CAA-admin site

`CAA-admin` is a web application that enables anyone to deal with the failed events that the `CAA-indexer` produces. To recap, the `CAA-indexer` will retry an event a finite number of times, but if it fails after this the event is moved to the persistent "failed" queue. Currently once it gets that far, we don't have any next steps.

`CAA-admin` uses `acid-state` to have a simple in-memory view at all events in this queue. It does so by listening on the failure queue, and consuming any fails (removing the queue). By consuming them, it stores them in its own database, that can later be queried. The Snap web framework is used to view the contents of this database in a web page, and users can select individual events and retry them - which simply redelivers them back to the `cover-art-archive` queue.