The Eurovision Song Contest is turning 70, with a few political clouds hanging over its glitter-drenched party. The annual musical extravaganza that has been likened to a pop music Olympics takes place in Vienna , Austria, May 12-16. It will see singers and bands from 35 countries compete onstage for the continent’s musical crown — but with some high-profile absentees who are boycotting to protest Israel’s participation. Here’s a guide to all things Eurovision: What it is, who to watch for and how to take part. A contest to unite a continent Eurovision is an international pop music competition founded in 1956 to test new live-broadcasting technology and foster unity after the trauma of World War II. From an initial seven contestants, it grew to include dozens of nations from across Europe, as well as farther-flung Israel and Australia. The contest is a campy yet heartfelt celebration of diversity, national pride and the joyous power of pop with a global fanbase.…