US Vice President Kamala Harris has thanked Joe Biden for his “historic leadership” during a surprise early appearance at the Democrats National Convention in Chicago on Monday. Harris made a brief unannounced appearance on stage, thanking Biden for his leadership ahead of his speech scheduled for later in the evening. “Joe, thank you for your historic leadership, for your lifetime of service to our nation, and for all you’ll continue to do,” Harris said. “We are forever grateful to you.” The four-day meeting is set to crown Harris as the Democratic Party’s choice to face Republican nominee Do…