[FIX] account: outstanding credit add button not shown in green

The "add" button shown in the outstanding credits menu in the
button right corner of a payment wasn't shown in green like the
other clickable elements.

opw-1907342

closes odoo/odoo#28908

addons/account/static/src/js/account_payment_field.js

@@ -128,6 +128,8 @@ var ShowPaymentLineWidget = AbstractField.extend({
      * @param {MouseEvent} event
      */
     _onOutstandingCreditAssign: function (event) {
+        event.stopPropagation();
+        event.preventDefault();
         var self = this;
         var id = $(event.target).data('id') || false;
         this._rpc({

addons/account/static/src/xml/account_payment.xml

@@ -14,7 +14,7 @@
                     <tr>
                     <t t-if="outstanding">
                         <td>
-                            <a title="assign to invoice" role="button" class="oe_form_field outstanding_credit_assign" t-att-data-id="line.id" style="margin-right: 10px;">Add</a>
+                            <a title="assign to invoice" role="button" class="oe_form_field btn btn-link outstanding_credit_assign" t-att-data-id="line.id" style="margin-right: 10px;" href="#">Add</a>
                         </td>
                         <td>
                             <span class="oe_form_field" style="margin-right: 30px;"><t t-esc="line.journal_name"></t></span>