Store and return invoice memo

controllers/api.js

@@ -197,6 +197,7 @@ router.post('/addinvoice', postLimiter, async function (req, res) {
     async function (err, info) {
       if (err) return errorLnd(res);
 
+      info.memo = req.body.memo;
       info.pay_req = info.payment_request; // client backwards compatibility
       await u.saveUserInvoice(info);
       await invoice.savePreimage(r_preimage);