Adjust weidget box so widget_subarea aligns with codecell

IPython/html/static/notebook/less/widgetarea.less

@@ -1,8 +1,12 b''
+div.widget_area {
+    page-break-inside: avoid;
+    .hbox();
+}
 
 /* This class is for the widget subarea inside the widget_area and after
    the prompt div. */
 div.widget_subarea {
     padding: 0.44em 0.4em 0.4em 1px;
     margin-left: 6px;
     .box-flex1();
 }