nginx.conf: Add security header policy

[iamimmanuelraj]

CSP and Perimission Policy when added can be extra good

But needs to be set as per usage so skipping it

Maybee after discussion we can set a minimal one

[iamimmanuelraj]

https://securityheaders.com/?q=immanuel.rt.gw&followRedirects=on https://www.ssllabs.com/ssltest/analyze.html?d=immanuel.rt.gw

Can see review here @mrrobot47