feat: add GA on the /welcome page on sourcegraph.com

Created by: dadlerj

NOTE: this change only affects sourcegraph.com, and only the /welcome page.

This is necessary because most of our inbound links go to "sourcegraph.com", which doesn't redirect to our about.sourcegraph.com or docs.sourcegraph.com subdomains when a user is unauthenticated. This means we're missing visibility of a huge chunk of non-app-usage traffic, and we can't track ad effectiveness.