In a large bowl, combine chicken and potatoes. Season with 2½ tsp salt and ½ tsp of pepper. In a small bowl, whisk together 3 Tbsp oil, harissa and cumin. Add to chicken and potatoes; toss to combine. Let it stand at room temperature for 30 minutes. In a medium bowl, combine leeks, lemon zest, ¼ tsp salt and remaining 1½ Tbsp oil. Preheat oven to 425°F. Arrange chicken and potatoes in a single layer on a large rimmed baking sheet. Roast 20 minutes. Toss potatoes lightly; scatter leeks over baking sheet. Roast 10-15 minutes or until the chicken is done and everything is golden and slightly crisped. In a small bowl, combine yogurt, garlic and the remaining ¼ tsp salt and ¼ tsp pepper. Spoon over chicken and vegetables. Scatter the herbs over top. Drizzle with oil and lemon juice.