LONDON, Aug 1 (Reuters) - British Prime Minister Keir Starmer warned social media companies that they must uphold laws that prohibit the incitement of violence online after misinformation around a fatal mass stabbing earlier in the week sparked violent scenes. A 17-year-old boy appeared in an English court on Thursday charged with the murder of three young girls in a knife attack at a summer dance class in Southport that has shocked the nation and sparked two nights of violent protests. The disturbances followed the rapid spread of false information on social media that the suspect in the stabbings was...