feat: add samesite GA cookie property to enable GA in iframe

[barnettx]

Problem

Google analytics currently does not work if a form is embedded on a different site.

Solution

• Set GA cookies to SameSite=None; Secure to allow iframe tracking
• Browsers have recently tightened cookie rules by ensuring the top-level domain (the domain in your browser navbar) is the same as the domain of the request before sending cookies in that request. In our case, the default SameSite=Lax for GA cookies meant that GA cookies were not being sent as the top-level domain eg. https://ogp-checkfirst-test-staging.netlify.app/ differed from the requests to http://www.google-analytics.com. SameSite=None tells the browser to ignore this same-domain check and attach the cookies on the GA request.
• As far as I can tell, setting SameSite=None has minimal security implications
• Because SameSite=None can only be set when the cookie is also Secure and the site is secure, this means that local testing will only work if using https
• References about SameSite cookies: https://web.dev/samesite-cookies-explained/
• https://medium.com/trabe/cookies-and-iframes-f7cca58b3b9e

Before & After

BEFORE:
iframe-form-no-GA-har.txt

AFTER:
iframe-forms-GA-har.txt

Tests

Tested by exposing my local formsg and embedding it in a test isomer site