Integrate cookie plugin into theme for better GraphCDN support

[drewbaker]

This is a plugin we currently use: https://github.com/MikhailRoot/samesite-cookie-manager/archive/refs/heads/master.zip

When using SSR sites with GraphCDN, we need to set the domain for cookies to .example.com, which isn't possible with Flywheel as they hard code the define() for COOKIE_DOMAIN to be the "WordPress Address (URL)".

So, I'd like to bring that code into the /functions directory, and upgrade it to use a settings field for the cookie domain. We have to be careful that we don't make it easy to allow the cookies to be set on a domain like "flywheelsites.com".

[drewbaker]

I've found a temp fix is to just find-replace COOKIE_DOMAIN in that plugin with the preferred domain works fine as a workaround.

[drewbaker]

@rsm0128 is working on this

[rsm0128]

@drewbaker I created PR #27 to arrange this.

[drewbaker]

Closed with 5bba037