WASHINGTON (Reuters) - U.S. Justice Department Special Counsel Robert Mueller has issued two subpoenas to a social media expert who worked for longtime Donald Trump adviser Roger Stone during the 2016 presidential election campaign. The subpoenas were delivered late last week to lawyers representing Jason Sullivan, a social media and Twitter specialist Stone hired to work for an independent political action committee he set up to support Trump, Knut Johnson, a lawyer for Sullivan, told Reuters on Tuesday. The subpoenas suggest that Mueller, who is probing Russian meddling in the 2016 U.S. presidential election, is focusing in part on Stone...