Dont show a user field on the front end

Closes joomla#149

libraries/cms/form/field/user.php

@@ -130,4 +130,29 @@ protected function getExcluded()
 	{
 		return explode(',', $this->element['exclude']);
 	}
+
+	/**
+	 * Transforms the field into an XML element and appends it as child on the given parent. This
+	 * is the default implementation of a field. Form fields which do support to be transformed into
+	 * an XML Element mut implemet the JFormDomfieldinterface.
+	 *
+	 * @param   stdClass    $field   The field.
+	 * @param   DOMElement  $parent  The field node parent.
+	 * @param   JForm       $form    The form.
+	 *
+	 * @return DOMElement
+	 *
+	 * @since 3.7
+	 * @see JFormDomfieldinterface::appendXMLFieldTag
+	 */
+	public function appendXMLFieldTag ($field, DOMElement $parent, JForm $form)
+	{
+		if (JFactory::getApplication()->isSite())
+		{
+			// The user field is not working on the front end
+			return;
+		}
+
+		return parent::appendXMLFieldTag($field, $parent, $form);
+	}
 }