ODESSA/SLAVIANSK, Ukraine, May 5 (Reuters) - Pro-Russian rebels shot down a Ukrainian helicopter in fierce fighting near the eastern town of Slaviansk on Monday, and Kiev drafted police special forces to the southwestern port city of Odessa to halt a feared westward spread of rebellion. Ukraine said the Odessa force, based on "civil activists", would replace local police who had failed to tackle rebel actions at the weekend. Its dispatch was a clear signal from Kiev that, while tackling rebellion in the east, it would vigorously resist any sign of a slide to a broader civil war. ...