This makes two changes to get rid of useless links presented to unauthenticated users.

1. Don't show the "Register" link if the site has disabled settings.BUILTIN_AUTH.  In that case it's just a redirect to the login page, so it's a redundant link.

2. Hide the navbar altogether if the site has enabled settings.REQUIRE_SITEWIDE_LOGIN.  None of the links in the navbar can be used until the user has logged in, so there's no reason to show it.

(There doesn't seem to be a consistent indentation style for template tags, so that may need some TLC before committing...)