Port ComputedPayment to mobx 4

Author: tanx

src/computed/payment.js

@@ -2,21 +2,23 @@
  * @fileOverview computed values that are used in payment UI components.
  */
 
-import { computed, extendObservable } from 'mobx';
+import { extendObservable } from 'mobx';
 import { toSatoshis, toAmountLabel, toLabel } from '../helper';
 
 const ComputedPayment = store => {
   extendObservable(store, {
-    paymentAmountLabel: computed(() =>
-      toLabel(store.payment.amount, store.settings)
-    ),
-    paymentFeeLabel: computed(() => toLabel(store.payment.fee, store.settings)),
-    paymentTotalLabel: computed(() => {
+    get paymentAmountLabel() {
+      return toLabel(store.payment.amount, store.settings);
+    },
+    get paymentFeeLabel() {
+      return toLabel(store.payment.fee, store.settings);
+    },
+    get paymentTotalLabel() {
       const { payment, settings } = store;
       const satAmount = toSatoshis(payment.amount, settings);
       const satFee = toSatoshis(payment.fee, settings);
       return toAmountLabel(satAmount + satFee, settings);
-    }),
+    },
   });
 };