Add user id to Snowplow package events

In this method, we seem to have access to current_user, but we're not passing it to Snowplow (::Gitlab::Tracking.event() method). We should add that attribute to the Snowplow event.

Long term, we might want to consider adding some validation that makes sure we always pass user in Snowplow events. We could start by adding this validation in shared Snowplow test examples.