The wikistories_contribution_event stream captures a variety of events from the Wikistories editing funnel: story_builder_open, add_frame, publish_success, and so on.
However, the stream doesn't have a unique identifier for each edit attempt. Without this, we cannot properly group events from a single journey through the funnel.
Let's add such an ID (call it contribution_attempt_id?). I would suggest using mw.user.generateRandomSessionId; there are existing IDs we could reuse (like the pageview token), but since we have no interest in joining this stream with another one, we should just be explicit about that and create a new one.