By Anna Koper and Kuba Stezycki WARSAW (Reuters) -Thousands of opposition supporters gathered outside Poland’s parliament on Thursday to protest against the new government’s changes to state media and the imprisonment of two former ministers convicted of abuse of power. The march reflects mounting tensions in the country as the new pro-European Union coalition government led by Donald Tusk tries to undo the policies of the previous nationalist Law and Justice (PiS) party administration. It also came as President Andrzej Duda, a PiS ally, said on Thursday he had started proceedings to pardon th…