Demnlus Johnson III waves and smiles to young kids and their parents as he asks a Nevin Community Center employee if they’ll be hosting a Halloween party this year. He’s at the community center — which sits across the street from his great grandparents’ home — speaking at a candidates’ forum about his plans for the city. Wearing a sharp, gray suit and delivering each answer with seemingly endless energy, Johnson, 25, has big plans for the city. He says…