The Justice Department is portraying  its new prosecution  of former FBI Director James Comey as not unlike dozens of other cases faced by individuals who have hurled threats at public officials.  But the Comey case may present unique challenges for prosecutors seeking to secure a conviction against the longtime foe of President Trump, legal experts say.  The former FBI director  faces two criminal charges  over allegations a photo of seashells he shared on social media last May that displayed the message “86 47” amounted to a threat on Trump’s life. He has not yet entered a plea but has denied any wrongdoing.   Acting Attorney General Todd Blanche said at a press conference announcing the charges this week that the DOJ has prosecuted dozens of cases involving threats against “all sorts of individuals” over the past year, from White House border czar Tom Homan to former President…