* Add $wgCookieSameSite, which controls the SameSite attribute for login
cookies. This will need to be set to "None" on WMF and other wikis
with a CentralAuth installation spanning multiple registrable domains.
* Add $wgUseSameSiteLegacyCookies, which causes a "legacy" cookie to be
sent without a SameSite attribute whenever a SameSite=None cookie is
sent. I used the prefix "ss0" since it's like SameSite version 0, and
that's shorter than "legacy". It's a prefix instead of a suffix to
avoid the need to update the VCL config which identifies cookie types
by their name suffix.
* Simplify WebRequest::getCookie() removing the unnecessary unicode
normalization. This was added by analogy with GET/POST, I don't
believe it was ever necessary for cookies.
* Add WebRequest::getCrossSiteCookie(), which implements the read side
of the legacy SameSite cookie support.
* Fix Doxygen formatting of the parameter list in
WebResponse::setCookie().
* To work around the lack of SameSite cookie support in PHP 7.2, emulate
setcookie() with header() where necessary.
Bug: T252236
Change-Id: I141ea114fea007a72a4f24bfc34dd81100854d68
f8a7a1ace8