Denmark’s Queen Margrethe marked her 50 years on the throne of Europe’s oldest ruling monarchy on Friday, with some of the planned festivities postponed for September due to the pandemic.The outbreak, which also stopped public celebrations for the popular monarch’s 80th birthday in 2020, forced several guests to stay away from the two-day celebrations. They included Margrethe’s younger sister, former queen Anne-Marie of Greece whose husband has tested positive for Covid-19.The Greek monarchy…