[#75962980] Check URL is blacklisted when reading from queue 

Check if a URL is blacklisted as we read it from the RabbitMQ queue to
prevent us from crawling blacklisted URLs.

Previously we would only check URLs that were extracted from a crawled
HTML document.
> Perhaps we could pair on it if you're in the office today?

I'll come and sit with you tomorrow morning. I've got some other ideas about how this could be done, but it's easier for us to look at them together.