Note: In a previous commit, I began making inc/mod/auth.php more modular with the check_login() function. Including it does NOT check mod login by default anymore like it does on vichan. You have to call check_login(). I've finally included it in inc/functions.php. If you have any custom pages that use inc/mod/auth.php, just including functions.php is enough now.

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Also: backports 351375185e5 (early 404)