Add a static instance variable reference to the BBP_Users_Admin class

[terresquall]

This is in reference to the BBP_Users_Admin class at includes/admin/users.php. There are some action hooks that reference the instance object itself in setup_actions(), but because the instantiation of the class is not assigned to a variable, there is no way to get a reference of the class.

To rectify this, we can add a static instance variable to it, then assign it in the construct function, like so:

/**
 * The bbPress users admin loader
 *
 * @since 2.0.0 bbPress (r2515)
 */
static $instance;
public function __construct() {
	self::$instance = $this;
	$this->setup_actions();
}

This makes it possible to remove the actions that are hooked by this class.