On Saturday, the area will host Urban Shield: Boston, a 24-hour training exercise that simulates large-scale public safety incidents in the metro-Boston area.  Urban Shield: Boston will begin at 8 a.m. Saturday and conclude at 8 a.m. Sunday. It will include personnel from the City of Boston, (including Boston Police, Fire, and EMS) Metro-Boston Homeland Security Region, the Commonwealth of Massachusetts, Massachusetts Bay Transportation Authority (MBTA), the Conference of…