Let $f(x)=x^2(1-x)^2$. What is the value of the sum $$f\left(\frac{1}{2019}\right)-f\left(\frac{2}{2019}\right)+f\left(\frac{3}{2019}\right)-\cdots+f\left(\frac{2017}{2019}\right)-f\left(\frac{2018}{2019}\right)$$