Create cookies with SameSite=Strict

[Joseph Reynolds]

I created a bmcweb issue/115 - https://github.com/openbmc/bmcweb/issues/115
to address the Chrome browser’s SameSite cookie changes 
(https://blog.chromium.org/2020/02/samesite-cookie-changes-in-february.html).

This was discussed in the 2020-02-19 meeting of the openbmc security 
workgroup, with minutes here:
https://docs.google.com/document/d/1b7x9BaxsfcukQDqbvZsU2ehMq4xoJRQvLxxsDUWmAOI

- Joseph