Add *CrawlerMessageItem.IsBlacklisted() function

Add a function `IsBlacklisted()` on the `CrawlerMessageItem` interface
that returns true if the item's URL is blacklisted.

We'll be using this in the next commit to check for blacklisted URLs
when reading messages from a RabbitMQ queue.
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.

Note that in the tests, we inspect the RabbitMQ queue to verify that the
message has been rejected. This is necessary because the channel
returned by `ReadFromQueue()` will always have a zero length (since the
message is rejected), so we can't perform any assertions on that
channel.
Merge pull request #49 from alphagov/75962980-blacklist-urls-read-from-queue

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