MOSCOW — Russian authorities have deployed busloads of riot police and security trucks around the church and cemetery where opposition leader Alexei Navalny, President Vladimir Putin’s most formidable critic, is due to be buried in Moscow. Rights groups warned mourners of possible arrests and interference. Navalny died suddenly at the age of 47 in the Polar Wolf prison colony in northern Russia — a death that his widow, Yulia Navalnaya, and other supporters have described as “murder,” but which investigators reported as due to “natural causes.”