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

db/migrate/20190701022101_add_trust_level_to_accounts.rb

@@ -0,0 +1,5 @@
+class AddTrustLevelToAccounts < ActiveRecord::Migration[5.2]
+  def change
+    add_column :accounts, :trust_level, :integer
+  end
+end