Add info about GoogleLogin to Special:Preferences

Like CentralAuth: Don't move content of Special:GoogleLogin
to Special:Preferences, "just" show information about the link
status and add a link to Special:GoogleLogin for information (already
linked) or to link wiki with Google account (not linked for now).

Bug: 68727
Change-Id: Ic33104b221dd415d509ffac81042187d127962c1