Fixed: OTRS does not protect against clickjacking (bug#10374).

Conflicts:
	CHANGES.md
	Kernel/Output/HTML/Standard/CustomerHeader.dtl
	Kernel/Output/HTML/Standard/CustomerHeaderSmall.dtl
	Kernel/Output/HTML/Standard/Header.dtl
	Kernel/Output/HTML/Standard/HeaderSmall.dtl
	Kernel/Output/HTML/Standard/Login.dtl
	Kernel/Output/HTML/Standard/PrintHeader.dtl