Add general modal functions

The two generic modals are defined in the base template, and are used
in various templates. So provide functions and remove duplicate code.

powerdnsadmin/templates/base.html

@@ -298,6 +298,18 @@
         }
       });
     });
+
+    function showErrorModal(message) {
+      var modal = $('#modal_error');
+      modal.find('.modal-body p').text(message);
+      modal.modal('show');
+    }
+
+    function showSuccessModal(message) {
+      var modal = $("#modal_success");
+      modal.find('.modal-body p').text(message);
+      modal.modal('show');
+    }
   </script>
   {% endif %}
 {%- endassets %}