support CentralAuthPostLoginRedirect (SUL2) hook

The CentralAuth extension on WMF wikis now redirects users after
account creation (due to $wgCentralAuthSilentLogin=true and other
configuration).  The create account success page never appears and so
the current code that adds Special:GettingStarted HTML to it
onBeforeWelcomeCreation doesn't run.

To address this, respond to the CentralAuthPostLoginRedirect hook to
redirect to Special:GettingStarted with ?postCreateAccount=true
(unless the original returntoquery contains a showGettingStarted=false
override).

When postCreateAccount is true, Special:GettingStarted outputs some of
what GettingStartedHooks::onBeforeWelcomeCreation and
SpecialUserlogin.php successfulCreation() would have done if CentralAuth
hadn't redirected:
* "Welcome first-time user" title.
* load accountCreation JS and CSS, some of which is redundant.
* output "Return to _original page_" link (which JS restyles).

To test the special page changes independent of the CentralAuth hook,
visit Special:GettingStarted with ?postCreateAccount=true,
with and without stressful returnto queries like
&returnto=D%C3%A9j%C3%A0%20vu%20%E5%8C%97%E4%BA%AC%20%26%20%22OK%22%20%27end%27
&returntoquery=foo%3Dbar%26baz%3D666

Change-Id: Ied765fdb6c6f007bb3b228bd20ce424606085596