make Browse Users list scrollable for smaller screens

On lower resolution screens, it's handy if the Browse Users list can scroll similar to Gradebook. The following adjustment has been tested as working:

In admin/user.php, change:
echo html_writer::table($table);

to:

echo html_writer::start_tag('div', array('class'=>'no-overflow'));
echo html_writer::table($table);
echo html_writer::end_tag('div');