Blocking message posting based on attachment checksum/hash if it was blocked before?

[slrslr]

Someone post illegal attachment, moderator click on the post and select ban&delete action. They create new ID and post same crap again. What about making a checksum/hash of that attachment after "ban&delete action" and save it into a blocked_attachments SQL table and no one is now allowed to add post witch such attachment? Maybe it can be made that such blocking will last limited time like 1 year. It can help mods a bit, but of course it can also silence valid attachment. But in some rooms benefit of this feature would be bigger than the downside, it can be optional too.

[mpretty-cyro]

I've added a brief section including the command needed to update the submodules as part of #962, this will be merged into master as part of the next release