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

Gemfile

@@ -58,6 +58,7 @@ gem 'idn-ruby', require: 'idn'
 gem 'kaminari', '~> 1.1'
 gem 'link_header', '~> 0.0'
 gem 'mime-types', '~> 3.2', require: 'mime/types/columnar'
+gem 'nilsimsa', git: 'https://github.com/witgo/nilsimsa', ref: 'fd184883048b922b176939f851338d0a4971a532'
 gem 'nokogiri', '~> 1.10'
 gem 'nsa', '~> 0.2'
 gem 'oj', '~> 3.7'