LONDON — Once upon a time, the British monarchy exerted a unique hold over the imaginations of millions of Americans, an interest that elevated its crown-bearing figureheads above the average A-lister or Hollywood hoi polloi. Lately, however, a succession of births, deaths and marriages in the royal family, and several high-profile scandals, have collided with the rise of an internet culture evermore obsessed with celebrity. The monthslong frenzy over the whereabouts of Catherine, Princess of Wa