TikTok fans are retaliating for Trump’s threats of banning the app, which is owned by China’s Bytedance Ltd. and is hugely popular in the U.S., especially among teens. The thought of taking away a key social and entertainment hub in the midst of the Covid-19 pandemic has led to outrage. “For Gen Z and Millennials, TikTok is our clubhouse and Trump threatened it,” said Yori Blacc, a 19-year-old TikTok user in California who joined in the app protest. “If you’re going to mess with us, we will mess with you.” Blacc said the movement gained steam Wednesday when a popular...