Implement an intent validation wrapper

[arrdem]

At present, there's nothing in the Katamari server stack which checks that a response conforms to the JSON response format including featuring an intent let alone a supported intent. Part of this is an unsolvable problem - unless the client reports its supported intents on every request the server can't check its work - but that whitelist should also be pretty stable at this point.

Implement a wrapper which checks all outgoing responses, ensuring that they at least feature a sane "intent" key.