Fix: Notifying Discord when a flag is created. (#4019)

Because:
* The flags controller was refactored with our move to project
submissions v2, but we forgot to switch the flag used with the discord
notifier method to an instance variable.

app/controllers/project_submissions/flags_controller.rb

@@ -41,7 +41,7 @@ def notify_discord_admins
     return unless Rails.env.production?
 
     DiscordNotifier.notify(
-      Notifications::FlagSubmission.new(flag)
+      Notifications::FlagSubmission.new(@flag)
     )
   end
 end