Deprecate HTTP_SEND_X_FRAME_OPTIONS

[TimWolla]

Embedding in frames:

• Reduces the security (by forcing SameSite=none on cookies and allowing clickjacking).
• Imposes bad UX (by not showing the actual URL in the URL bar).
• Interoperates especially badly with Firefox' scoped cookie jars, because WoltLab Suite will see differing cookies depending on whether the site is embedded in a frame or not.