- Prevents authenticated responses being visible when back button
pressed in browser.
- Previously, 'no-cache, private' was added by default by Symfony which
would have prevents proxy cache issues but this adds no-store and a
max-age option to also invalidate all caching.
Thanks to @haxatron via huntr.dev
Ref: https://huntr.dev/bounties/6cda9df9-4987-4e1c-b48f-855b6901ef53/
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.