In a bowl, combine the yogurt, olive oil, chicken, lemon juice, garlic, shallots, paprika, oregano, chili flakes, and a large pinch each of salt and pepper. Let marinate for 15 minutes at room temperature or up to overnight in the refrigerator.Preheat the oven to 425° F. Arrange the chicken on a baking sheet. Bake 15 minutes, toss and bake another 5-10 minutes, or until cooked through. Switch the oven to broil. Broil 1-2 minutes, until the chicken chars on the edges (watch carefully) Meanwhile, make the Tzatziki. Combine the feta, 1/4 cup yogurt, garlic, and lemon juice in a blender and blend until creamy. Stir in 1/3 cup more yogurt, the cucumbers, and a pinch of salt and red pepper flakes.To assemble, stuff/spread each pita with tzatziki, then lettuce, and chicken. Add red onions and herbs. Drizzle over more tzatziki and if desired, harissa (recipe in notes).