Fox News, which is hosting the first Republican debate of the 2016 campaign, will require participants to place in the top 10 in an average of the five most recent national polls in the run-up to the event, according to details obtained by The Washington Post Thursday. The standard will winnow what is expected to be a field of 16 or more GOP presidential candidates by the Aug. 6 event in Cleveland. Determining which contenders will get to participate in the official forums sanctioned by the Republican National Committee has been a thorny challenge for the cable news network and...