[FIX] payment_stripe: correctly compute reference number

As explained in 15f27b26, the transaction information should be computed when
hitting the "pay now" button, not at page rendering (e.g. in case of users going
back to previous page).

Stripe do not work as typical providers as it does not redirect to the provider
page but opens a popup.
The route /shop/payment/transaction/<id> needs to be called manually.

addons/payment_stripe/static/src/js/stripe.js

@@ -39,7 +39,23 @@ odoo.define('payment_stripe.stripe', function(require) {
         if(!$(this).find('i').length)
             $(this).append('<i class="fa fa-spinner fa-spin"/>');
             $(this).attr('disabled','disabled');
-        handler.open();
-        e.preventDefault();
+
+        var $form = $(e.currentTarget).parents('form');
+        var acquirer_id = $(e.currentTarget).parents('div.oe_sale_acquirer_button').first().data('id');
+        if (! acquirer_id) {
+            return false;
+        }
+
+        ajax.jsonRpc('/shop/payment/transaction/' + acquirer_id, 'call', {}).then(function (data) {
+            $form.html(data);
+            handler.open({
+                name: $("input[name='merchant']").val(),
+                description: $("input[name='invoice_num']").val(),
+                currency: $("input[name='currency']").val(),
+                amount: $("input[name='amount']").val()*100
+            });
+            e.preventDefault();
+        });
+
     });
 });