Updated default value for APP_URL so that the startsWith call is not
passed null, since that causes deprecation notice in PHP8.1.
Would show when APP_URL was not set, adding extra confusiion.
Added iframe CSP headers with configuration via .env.
Updated session cookies to be lax by default, dynamically changing to
none when iframes configured to allow third-party control.
Updated cookie security to be auto-secure if a https APP_URL is set.
Related to #2427 and #2207.