By Amanda Ferguson and Paul Sandle BELFAST/LONDON (Reuters) -Northern Ireland’s Democratic Unioinist Party (DUP) refused on Monday to form a new power-sharing administration until post-Brexit trading rules are overhauled, rebuffing calls from London and Dublin to quickly go back into government. Sinn Fein, the former political wing of the Irish Republican Army (IRA), overtook the DUP to win the most seats at elections to the region’s devolved assembly at the weekend, the first time a party seeking to leave the United Kingdom came out on top. Under the terms of a 1998 peace deal that mostly end…