WASHINGTON, June 30 (Reuters) - A federal appeals court heard oral arguments on Monday in what is likely to be a decisive lawsuit over whether President Donald Trump's administration can use an 18th-century law to speed up deportations.The 5th U.S. Circuit Court of Appeals is weighing whether Venezuelan migrants being held at the Bluebonnet Detention Facility in Anson, Texas, can be deported under the Alien Enemies Act.American Civil Liberties Union lawyer Lee Gelernt, arguing for the detainees, said the 1798 law was meant to be used during wartime, and "the government itself is saying we aren't in a military conflict"...