cloud: Deploy a dead-letter topic for submission queues #536
Description
The Google Cloud Pub/Sub system supports assigning a "dead-letter" topic to a subscription, which receives all messages that failed delivery despite trying it for some time. This could be useful for collecting messages which failed delivery for one reason or another, and helping the CI systems as well as KCIDB developers debug submission issues.
At the moment we drop any messages which fail JSON schema validation, perhaps we should be putting them into such "dead-letter" topic ourselves as well, immediately, instead of NACK'ing them and letting Pub/Sub retry them uselessly.