cookie-jar: use base domain to decide if cookie is third party

Our third-party cookie blocking is too strict. Safari is the only major
browser that blocks third-party cookies by default, so the only way for
SOUP_COOKIE_JAR_ACCEPT_NO_THIRD_PARTY to be web compatible is for it to
match Safari as closely as possible. And Safari does not appear to
consider the full domain when deciding whether to block a third-party
cookie, it only considers the base domain. Reports indicate that Firefox
and Chrome behave similarly if the user chooses to block third-party
cookies in those browsers.

This fixes, in particular, notifications on google.com domains.

https://bugzilla.gnome.org/show_bug.cgi?id=792130