Fixes dpa_is_single_user_achievements for custom endpoints

Hey Paul,

I'm back on deck and into our achievements project again. I just stumbled across a bug where my achievements weren't showing in BuddyPress. We're using a custom endpoint called 'challenges'. This fixes the templates for custom endpoints. Cheers!

P.S. I'll try and get some sample code to you regarding the leaderboard tomorrow too :grin: