Over the past few years, Mastodon has become the model for a friendlier kind of social network, promising to keep out the hateful or ugly content that proliferates on larger and more centralized networks. Journalists hailed it as “Twitter without Nazis” and for years, it’s generally lived up to that promise. But last week, the social network Gab migrated to Mastodon — and Mastodon’s admins have been forced to deal with the internet’s Nazi problem head-on.