Adding HSTS header

Same as https://github.com/sebsauvage/ZeroBin/pull/70

Adding the `Strict-Transport-Security` header, you locks the traffic on
https protocol for a day (86400 seconds). If the server is not
https-capable, this security layer is simply ignored by browser. See:
https://developer.mozilla.org/en-US/docs/Security/HTTP_Strict_Transport_
Security