Add Logout Function

[j0nezz]

Especially for debugging, a logout function would be handy.
As the cookie is HTTP only, it can only be deleted by overwriting the cookie with max-age: 0

See https://developer.mozilla.org/en-US/docs/Web/HTTP/Headers/Set-Cookie

Max-Age= Optional
Number of seconds until the cookie expires. A zero or negative number will expire the cookie immediately. If both Expires and Max-Age are set, Max-Age has precedence.