There’s no getting around it. “Mamma Mia 2” is well worth the decade-long wait. The sequel to the exhilarating original is a narrative necessity that helps viewers understand the present by visiting the past. Picking up not too many years after the events of the first film, Donna has passed away and Sophie is reopening her late mother’s hotel. There is also conflict in her relationship with Sky as the couple disagrees about where their future should be.