Jan Zizka, a beloved 15th-century Czech folk hero, looms large in the nation’s collective consciousness, a skilled, ingenious, one-eyed warrior who led peasants and rebels into battles that he never lost. In Prague, they’ve erected one of the largest bronze equestrian statues in the world in his honor, and now, the most expensive Czech film ever made, “Medieval,” depicts his early years, with the intense actor Ben Foster taking on the role of Zizka. “Medieval” is written and directed by Czech filmmaker, actor and stuntman Petr Jakl, who also represented the country in judo in the 2000 Olympics…