Milo Yiannopoulos, the incendiary writer and commentator who helped make Breitbart News a leading organ of the “alt-right,” resigned from the news organization Tuesday after a video of him endorsing pedophilia resurfaced online over the weekend. Yiannopoulos — known simply as MILO in Breitbart’s own coverage of him — has been a flame-throwing provocateur whose writing has offended women, Muslims, blacks and gay people ever since former Breitbart executive chairman Stephen K. Bannon hired him as a senior editor in 2014.