ATHENS, June 29 (Reuters) - With backing from a politician jailed for leading a party declared a criminal gang, Greek far-right groups swept up over 12% of the vote in Sunday's election, mirroring the rise of populist and ultra-nationalist politicians across Europe. The surge of three parties with their ultra-nationalist views - including 'Spartans' which barely registered in polls until Ilias Kasidiaris from the banned Golden Dawn party endorsed it from his prison cell -- could swing public debate at home and prove a springboard in European elections. While the centre-right New Democracy party of Kyriakos Mitsotakis stormed to victory...