DEV: Make `:send_system_message` arguments strings (#15851)

app/models/post.rb

@@ -574,7 +574,7 @@ class Post < ActiveRecord::Base
         5.seconds,
         :send_system_message,
         user_id: user.id,
-        message_type: message,
+        message_type: message.to_s,
         message_options: options
       )
     end

app/models/reviewable_flagged_post.rb

@@ -321,7 +321,7 @@ private
     Jobs.enqueue(
       :send_system_message,
       user_id: post.user_id,
-      message_type: :flags_disagreed,
+      message_type: "flags_disagreed",
       message_options: {
         flagged_post_raw_content: post.raw,
         url: post.url