Introduce jquery formvalidator for com_users

#### Executive summary

This PR converts the form validation on com_users to use plain jquery (no mootools call on every form ).
Also NO MORE INLINE SCRIPTS!

#### Testing

1. Apply first PR #4888 (!important)
2. Apply this PR
3. In the admin area go to com_users and try to submit any form.

If no javascript errors are logged in your browser and the functionality remains the same your test is a pass in any other case please report the errors here

Is this ok?
```
$script = '
jQuery(document).ready(function() {
	Joomla.submitbutton = function(task) {
		if (task == "groups.delete")
		{
			var f = document.adminForm;
			var cb="";';
foreach ($this->items as $i => $item)
{
	if ($item->user_count > 0)
	{
		$script .= '
			cb = f["cb"+' . $i . '];
			if (cb && cb.checked) { 
				if (confirm(Joomla.JText._("COM_USERS_GROUPS_CONFIRM_DELETE"))) {
					Joomla.submitform(task);
				}
				return;
			}';
	}
}
$script .= '
		}
	Joomla.submitform(task);
	}
});';

JFactory::getDocument()->addScriptDeclaration($script);
```