Refactor: Extract RabbitMQ management client & provide fakes

[coro]

This came from my looking into #88. Our only tests that touched the controller also used a real RabbitMQ, so it was not possible to control the responses from the RabbitMQ API in these tests, and force the API to respond with a 401.

This extracts all of the rabbithole client logic into a factory, which creates an object providing all of the rabbithole methods that we use in our controllers. This also adds counterfeiter methods to create fakes for all of these methods, allowing us to write more comprehensive integration tests at the per-controller level.

In the actual binary, we use a factory to generate a real rabbithole client. In the tests, we can now create our own mock factory which creates a fake RabbitMQ management client, which can return whatever we choose in the tests.

I will use this framework to address #88 by writing a test to return an error response from rabbithole, and triggering the panic hit in that error.

Additional considerations

This means we will have to add to the interface and regenerate the fake code whenever we use a new rabbithole method. We can always add a make target for the autogen side of this if needs be.