Added iframe CSP, improved session cookie security

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.

app/Config/session.php

@@ -1,5 +1,7 @@
 <?php
 
+use \Illuminate\Support\Str;
+
 /**
  * Session configuration options.
  *
@@ -69,7 +71,8 @@ return [
     // By setting this option to true, session cookies will only be sent back
     // to the server if the browser has a HTTPS connection. This will keep
     // the cookie from being sent to you if it can not be done securely.
-    'secure' => env('SESSION_SECURE_COOKIE', false),
+    'secure' => env('SESSION_SECURE_COOKIE', null)
+        ?? Str::startsWith(env('APP_URL'), 'https:'),
 
     // HTTP Access Only
     // Setting this value to true will prevent JavaScript from accessing the
@@ -80,6 +83,6 @@ return [
     // This option determines how your cookies behave when cross-site requests
     // take place, and can be used to mitigate CSRF attacks. By default, we
     // do not enable this as other CSRF protection services are in place.
-    // Options: lax, strict
-    'same_site' => null,
+    // Options: lax, strict, none
+    'same_site' => 'lax',
 ];