Add a spam check (#11217)

* Add a spam check

* Use Nilsimsa to generate locality-sensitive hashes and compare using Levenshtein distance

* Add more tests

* Add exemption when the message is a reply to something that mentions the sender

* Use Nilsimsa Compare Value instead of Levenshtein distance

* Use MD5 for messages shorter than 10 characters

* Add message to automated report, do not add non-public statuses to
automated report, add trust level to accounts and make unsilencing
raise the trust level to prevent repeated spam checks on that account

* Expire spam check data after 3 months

* Add support for local statuses, reduce expiration to 1 week, always create a report

* Add content warnings to the spam check and exempt empty statuses

* Change Nilsimsa threshold to 95 and make sure removed statuses are removed from the spam check

* Add all matched statuses into automatic report

app/lib/activitypub/activity/create.rb

@@ -41,6 +41,7 @@ class ActivityPub::Activity::Create < ActivityPub::Activity
 
     resolve_thread(@status)
     fetch_replies(@status)
+    check_for_spam
     distribute(@status)
     forward_for_reply if @status.distributable?
   end
@@ -406,6 +407,18 @@ class ActivityPub::Activity::Create < ActivityPub::Activity
     Account.local.where(username: local_usernames).exists?
   end
 
+  def check_for_spam
+    spam_check = SpamCheck.new(@status)
+
+    return if spam_check.skip?
+
+    if spam_check.spam?
+      spam_check.flag!
+    else
+      spam_check.remember!
+    end
+  end
+
   def forward_for_reply
     return unless @json['signature'].present? && reply_to_local?
     ActivityPub::RawDistributionWorker.perform_async(Oj.dump(@json), replied_to_status.account_id, [@account.preferred_inbox_url])