[FIX] web: discarding a new record in a modal closes the modal

Discarding a new record triggers the 'history_back' action.  This makes
sense if the record was discarded from a normal form view, but not so
much when in a modal, where the expected behaviour is to close the
dialog.

This commit overrides the function in the action manager which handles
the switch to the previous view. It checks whether a dialog window is
open, in which case it simply closes it.

addons/web/static/src/js/chrome/action_manager.js

@@ -420,6 +420,10 @@ var ActionManager = Widget.extend({
         return this.inner_widget;
     },
     history_back: function() {
+        if (this.dialog) {
+            this.dialog.close();
+            return;
+        }
         var nbViews = this.inner_action.get_nb_views();
         if (nbViews > 1) {
             // Stay on this action, but select the previous view