Add OpenID SSO support.

Configuring OPENID_SSO in gerrit.config will allow the admin
to specify an SSO entry point URL so that users clicking on
"Sign In" are sent directly to that URL.

Change-Id: I361b7b74a6f34a199578620540be31d1df0c5fef
Bump openid4java dependency to 0.9.8

Authentication is security-sensitive, so best to make use of the
latest available stable version.

Change-Id: I70904821a61408b84fcaef5e818ff07fd1f9aecb