LONDON — Who looks at Islamist extremist content online? A new study released Tuesday by the Policy Exchange think tank based in London, whose reports often inform government policy in Britain, ranked the top consumers of propaganda produced by the Islamic State, by country of origin, as measured by clicks. The top consumers of Islamic State videos were from Turkey, the United States, Saudi Arabia, Iraq and Britain, which registered the largest number of clicks in Europe. There is bad news and less bad news in the report, “The New Netwar,” from scholars led by Martyn Frampton at Queen Mary University of London. Bad, in the sense that the Islamic State is pumping out more material than ever.