Spam-protection

We need something like MT-Blacklist: the ability to define URL patterns that flag a page and/or comment as spam.

It shouldn't be too hard to do - we already track URL links. The UI will need some thought though (what do you do if you define a URL as spam, and it's in a page? Revert the page back to the first version without the URL? What if the URL's been there a while and survived several edits?)