show loading on braintree

src/scss/styles.scss

@@ -529,6 +529,11 @@ app-user-billing {
     }
 }
 
+#bt-dropin-container {
+    background: url('../images/loading.svg') 0 0 no-repeat;
+    min-height: 50px;
+}
+
 .braintree-placeholder, .braintree-sheet__header {
     display: none;
 }