Add the ability to watch/blacklist images

[makyen]

On numerous occasions, we have spammers and trolls persistently posting specific images. It would be helpful to be able to watch/blacklist such images.

General outline:
SD identifies URLs for images and links to known image hosting sites in a post and fetches those resources. A checksum is then calculated over the response and compared to a list of blacklisted or watches checksums.

Some thoughts:

• There would need to be (un)watch/blacklist commands for image URLs, which SD would then fetch and checksum.
• Directly (un)watching/blacklisting specific checksums should also be supported.
• The source URL and/or SE/MS post should probably also be recorded in the watch/blacklist file to make it easier for humans to find the source information.
• It would be good to have (un)watch/blacklist commands that accepted an SE or MS post which, effectively, watched/blacklisted all images in that post.
• It would be good to have information commands that showed the URLs for detected resources in a SE/MS post and their checksums, along with ones which did the same for URLs.

Drawbacks:

• SD would be fetching lots more resources for every post. This would add additional delay to scanning. This could be mitigated by having the fetching be asynchronous, but that's not a small task, given that there are other things that also should be asynchronous.
  • It would expose the IP address which SD is using to the sites hosting those resources.