Britain’s political leaders sparred Sunday over who is responsible for the early release of a convicted extremist who launched a stabbing attack in central London that left two dead and injured three. The argument centers over the early release from prison of Usman Khan, who served roughly half his sentence before being set free. After a one-day pause out of respect for victims, the Friday attack is dominating the political scene as the Dec. 12 election nears, shifting the focus, at least for the moment, from Brexit and the National Health Service to issues of security and criminal justice.