Fix central session usage in Special:CentralAutoLogin

The central session expires when the user's browser is closed.
Special:CentralAutoLogin should check for this, and take appropriate
measures if the keys it had stored in the session are no longer present.

This fixes various PHP warnings and makes it better about honoring the
"remember me" checkbox from the original login.

Change-Id: I1f551fed312af7e70c5899d92e7f5726710f6f6d
Merge "Fix central session usage in Special:CentralAutoLogin"