BRUSSELS (Reuters) - European Union leaders gave their formal approval on Friday to the opening of a second phase of Brexit negotiations with Britain, focusing on a transition period and future trading relationship.  EU leaders agree to move on to the second phase of Brexit talks,  European Council President Donald Tusk tweeted during a Brussels summit that he was chairing.   Congratulations PM @Theresa,  he added, a day after leaders acknowledged British Prime Minister Theresa May s efforts to conclude an outline divorce settlement by giving her a round of applause. 