Allow developers to set the URL that users will be directed to after they log out

[CaMer0n]

Motivation

At present, e107 always sends the user to the home page after logging out. (even from the admin area)
This is not always a desired outcome for the developer.

Proposed Solution

Allow the developer to set the URL for after logout using the existing method:

e107::getRedirect()->setPreviousUrl()

To date, this has only been utilized for setting the URL for after login, as found in login.php:

$prev = e107::getRedirect()->getPreviousUrl();

if(!empty($prev))
{
	e107::redirect($prev);
	exit();
}