KINGS BAY, Georgia (Reuters) - The 2003 invasion of Iraq was a mistake that has boosted the ranks and morale of extremist Islamic groups and made them a greater threat to U.S. interests, former President Jimmy Carter said on Friday. "We have not lessened the strength of terrorists around the world and have not lessened but have increased both the number and the fervor and the organizational capabilities of terrorists," Carter said in a news conference at this U.S. naval base in southeastern Georgia. Carter, the winner of the Nobel Peace Prize in 2002 and one of the fiercest U.S....