Fixes #6999 - protect user logout against CSRF requests (CVE-2014-3590)

To avoid CSRF, logout is changed to be a POST request so
protect_from_forgery checks the CSRF token. However, in Rails 3 the only
strategy available is to nullify the session of the attacker.
We modify this behavior to raise a Foreman Exception.
This issue is probably worth revisiting on the update to Rails 4 as
throwing an exception is a valid strategy again.