Natalie Portman disappears into Camelot with her astonishing portrayal of Jacqueline Kennedy in “Jackie.” Portman, director Pablo Larraín and screenwriter Noah Oppenheim provide an unusual glimpse into what life must have been like for Jackie in the week following the assassination of her husband, President John Fitzgerald Kennedy, as she juggled two small grieving children, her own melancholy and the mourning of a nation.