Add Strict-Transport-Security header

[php-coder]

See for details:

• https://httpsecurityreport.com/best_practice.html#hsts
• https://securityheaders.io
• https://scotthelme.co.uk/hsts-the-missing-link-in-tls/
• https://jvns.ca/blog/2017/04/30/using-strict-transport-security/
• https://habrahabr.ru/post/320164/ (in Russian)