To protect the backend from click jacking attacks
a HTTP header needs to be sent, which prevents
embedding backend pages in an iframe on domains
different than the one used to access the backend.

All recommended browsers respect this header
and prevents the backend page to be shown in an
iframe, so we do not need to implement further
JavaScript frame busting solutions.

Resolves: #54201
Documentation: #57144
Releases: 6.2
Change-Id: Ic83cae4917bb62ff8fe8b55a947ace7dba86d223
Reviewed-on: https://review.typo3.org/28601
Reviewed-by: Christian Kuhn
Reviewed-by: Markus Klein
Tested-by: Markus Klein
Reviewed-by: Wouter Wolters
Tested-by: Wouter Wolters
Reviewed-by: Ernesto Baschny
Tested-by: Ernesto Baschny