Add a check for the scheduled publishing queue.

From time to time, an item will be scheduled for publishing, but the
code to add it to the Sidekiq queue will fail. We should have monitoring
for when this happens, so we can give it a nudge.
Recover from Mongo/Redis failures in healthcheck.

If the health check returns a non-200 response, our monitoring
currently treats this as the entire app being broken, so we should
recover from errors wherever possible.