Fix Earned Achievements widget handling for achievements with spaces and special characters in their "single_name"; Previously, an achievement like "My Badge" could be selected, but that didn't match the widget() method which checks against post_type, where as "Badge" would match post_type since there were no spaces

@rzen We may want to figure out a migration path for widget options here, we could check if `in_array( $achievement[ 'single_name' ], $set_achievements )` here too, what do you think? There could be cases where single_name is the same as $achievement_slug, maybe an edge case but possible.