[core] Fix Login redirection vulnerability

After a successful login, HUE allows redirection to an arbitrary site by modifying the site "<malicious.com>" in the URL: http://<my hue site>/accounts/login/?next=http://<malicious.com>.
Since the attacker is allowed to fully control the name of the destination site, this could be used to mount a phishing attack against http://<my hue site>'s users, such as requiring the users to re-enter their credentials at the new site.

This patch sets redirect_whitelist configuration default value to "^\/.*$”, and limits all redirections to the same host where HUE is running by default to prevent phishing attacks.