How many secrets can one family have? Filmmaker Emerald Fennell returns with “Saltburn,” a darkly comedic drama centered on a group of wealthy Oxford students who escape to a lush estate for the summer. A follow-up to her critically acclaimed effort “Promising Young Woman,” the upcoming project sees a hapless college student search for a welcoming clique; what he discovers is another student offering a different world and the prospect of an irresistible invitation.