Return HTTP 200 instead of HTTP 204 for payment notifications

Redsys are a bunch of … something: they **only** recognize HTTP 200
as success; HTTP 204 apparently is an error for they, and don’t get
me started in not understanding what to do with an HTP 301.

Let’s just give in, and relax.

pkg/payment/public.go

@@ -251,7 +251,7 @@ func handleNotification(w http.ResponseWriter, r *http.Request, user *auth.User,
 				_ = sendEmail(r.Context(), conn, payment, company, user.Locale) /* shrug */
 			default:
 			}
-			w.WriteHeader(http.StatusNoContent)
+			w.WriteHeader(http.StatusOK)
 		default:
 			httplib.MethodNotAllowed(w, r, http.MethodPost)
 		}