Use new config system for Discord Rich Presence option

This doesn't feel like a mirror change to me.

Source/Core/UICommon/DiscordPresence.h

@@ -4,9 +4,14 @@
 
 #pragma once
 
+#include "Common/Config/Config.h"
+
 namespace Discord
 {
 void Init();
 void UpdateDiscordPresence();
 void Shutdown();
+void SetDiscordPresenceEnabled(bool enabled);
 }  // namespace Discord
+
+extern const Config::ConfigInfo<bool> MAIN_USE_DISCORD_PRESENCE;