By Jessica Frank-Keyes Boris Johnson’s prime-time interview with the BBC has been cancelled after briefing notes were shared with him by presenter Laura Kuenssberg “by mistake”. The former Prime Minister was meant to be interviewed by the former political editor in his first major sit-down since leaving office, to be broadcast on BBC One at 7.30pm on Thursday, with topics expected to include Brexit, the Covid-19 pandemic and Partygate. But Kuenssberg, who now hosts the BBC’s flagship Sunday morning political interview show, revealed she mistakenly sent the briefing notes to him “in a message m…