Calypso Admin Plans Page: Allow coupon query param (#95630)

client/my-sites/plans/controller.jsx

@@ -32,14 +32,22 @@ export function plans( context, next ) {
 		return productSelect( '/plans' )( context, next );
 	}
 
+	// Emails rely on the `discount` query param to auto-apply coupons
+	// from the Calypso admin plans page. The `/start` onboarding flow
+	// plans page, however, relies on the `coupon` query param for the
+	// same purpose. We handle both coupon and discount here for the time
+	// being to avoid confusion. We'll probably consolidate to just `coupon`
+	// in the future.
+	const withDiscount = context.query.coupon || context.query.discount;
+
 	context.primary = (
 		<Plans
 			context={ context }
 			intervalType={ context.params.intervalType }
 			customerType={ context.query.customerType }
 			selectedFeature={ context.query.feature }
 			selectedPlan={ context.query.plan }
-			withDiscount={ context.query.discount }
+			withDiscount={ withDiscount }
 			discountEndDate={ context.query.ts }
 			redirectTo={ context.query.redirect_to }
 			redirectToAddDomainFlow={