web/satellite/vuetify-poc: hide 'Add coupon' button when coupon features are disabled

Hide button if feature flag is set to false.

Issue:
https://github.com/storj/storj/issues/6547

Change-Id: I0e13bacfe6a8651ebc573abc3f23a749821d63a5

web/satellite/vuetify-poc/src/views/Billing.vue

@@ -91,6 +91,7 @@
                         <v-card
                             v-else-if="coupon"
                             :title="`Coupon / ${coupon.name}`"
+                            height="100%"
                             :subtitle="`${isCouponActive ? 'Active' : 'Expired'} / ${couponExpiration}`"
                             rounded="xlg"
                         >
@@ -107,6 +108,7 @@
                                 <v-divider class="my-4" />
 
                                 <v-btn
+                                    v-if="couponCodeBillingUIEnabled"
                                     variant="outlined"
                                     color="default"
                                     size="small"