Change spam check to apply to local accounts and add a threshold (#11806)

Instead of detecting spam on first duplicate message, add a
threshold of 5 such messages to reduce false positives

app/services/process_mentions_service.rb

@@ -33,6 +33,7 @@ class ProcessMentionsService < BaseService
     end
 
     status.save!
+    check_for_spam(status)
 
     mentions.each { |mention| create_notification(mention) }
   end
@@ -61,4 +62,8 @@ class ProcessMentionsService < BaseService
   def resolve_account_service
     ResolveAccountService.new
   end
+
+  def check_for_spam(status)
+    SpamCheck.perform(status)
+  end
 end