oauthclient: cross-site request forgery fix
- Fixes a CSRF issue in oauthclient in which the next parameter to "/oauth/login/app/?next=" could be used by an attacker to redirect the end-user to an external URL.
- Adds use of OAuth 2.0 state parameter for all oauthclients for further CSRF protection.
Reported-by: rcpeters <info@rcpeters.com>
Signed-off-by: Lars Holm Nielsen <lars.holm.nielsen@cern.ch>