fix: improve individual discount loading

lib/new-admin/graphql/resolvers/loyalty.resolver.js

@@ -1,6 +1,16 @@
+const DataLoader = require('dataloader')
+
 const loyalty = require('../../../loyalty')
+const { getSlimCustomerByIdBatch } = require('../../../customers')
+
+const customerLoader = new DataLoader(ids => {
+  return getSlimCustomerByIdBatch(ids)
+}, { cache: false })
 
 const resolvers = {
+  IndividualDiscount: {
+    customer: parent => customerLoader.load(parent.customerId)
+  },
   Query: {
     promoCodes: () => loyalty.getAvailablePromoCodes(),
     individualDiscounts: () => loyalty.getAvailableIndividualDiscounts()