FEATURE: Silence watched word (#13160)

This is a new type of watched word to replace auto_silence_first_post_
regex site setting.

app/models/watched_word.rb

@@ -10,6 +10,7 @@ class WatchedWord < ActiveRecord::Base
       flag: 4,
       replace: 5,
       tag: 6,
+      silence: 7
     )
   end