BERLIN (Reuters) - Online fashion retailer Zalando said it expected sales growth to slow to 15-20% in 2020 after it reported a strong final quarter to 2019, boosted by a strong performance in the "cyber" week around the Black Friday discount day.Fourth-quarter sales rose 19.5% to 2 billion euros (1.7 billion pounds), while adjusted earnings before interest and taxation came in at 110.4 million euros, both ahead of average analyst forecasts.While it expects sales growth to come in at 15-20% in..