At 16 years old, the Nickelodeon star has turned reality-TV and YouTube fame into a global brand. Will she be the traditional teen idol machine’s saving grace? — By Brittany Spanos On a nondescript, cactus-lined street in Sherman Oaks, California, it’s hard to miss the only house with a unicorn-drawn-carriage and a custom BMW painted like a rainbow race car on its curb. Inside the house, singer, actress, and YouTube superstar JoJo Siwa is covered in flour, sprinkles…