Rework CentralAuth login flow

Changes in this mega-patch are:
* "Silent login" option: skip the login success screen if
  $wgCentralAuthSilentLogin is set. This may eventually become
  non-optional.
* Check against the central wiki for whether the user is logged in. If
  so, the p-personal bar will be rewritten, and (if the user has certain
  preferences set) a notification will be presented.
* Non-JS-based check against the central wiki for whether the user is
  logged in, for users without JavaScript enabled.