Moved Tinybird tracker token to the analytics service

[cmraible]

This moves the TINYBIRD_TRACKER_TOKEN environment variable out of Ghost's configuration, and passes it directly to the analytics service.

The Tinybird tracker token grants write access to the analytics_events datasource in Tinybird. Before we had the analytics service sitting between Ghost and Tinybird, Ghost needed to send this token with each tracking event request, or else Tinybird would return a 401 error. This isn't very secure, since the tracker token is exposed in plain text in the network tab whenever visiting a Ghost site's frontend.

Now that we have the analytics service sitting between Ghost and Tinybird, we can add the token to each request there, which means Ghost no longer needs to pass the token with each request, and in fact doesn't need to use the tracker token at all.