SAN FRANCISCO — A prominent white nationalist is suing Twitter for banning his accounts at a time when social networks are trying to crack down on hateful and abusive content without appearing to censor unpopular opinions. Jared Taylor filed the lawsuit Tuesday in state court in San Francisco, marking the latest legal challenge filed by right-wing groups and figures banned from social media sites. Taylor is the founder of the Virginia-based New Century Foundation, an IRS-classified charity that operates the American Renaissance online magazine.