fixes #7805 - Add several security related HTTP headers - security hardening.

This commit uses secure_headers gem and configures several HTTP
security related headers to be sent by server:
* Content Security Policy
* HTTP Strict Transport Security
* X-XSS-Protection
* X-Frame-Options
* X-Content-Type-Options
All of these enable browser protections on client side and make
exploitation of common web flaws harder.