Activist investor Jana Partners has teamed up with football star Travis Kelce to push Six Flags Entertainment to improve its marketing and customer experiences, the investor said, after disclosing a 9% stake in the company.Jana Partners’ Scott Ostfeld announced the position at the 13D Monitor Active Passive-Investment Summit in New York on Tuesday, saying he has partnered with Kelce and two business industry executives to pressure the company to improve its performance.Shares of Six Flags shot up 17% on Tue ...