Retrieves user actions

Lists user actions (form submissions). Can be filtered by user_id, page_id, or both. To get custom form responses for event RSVPs, first get the event's event_page_id from GET /events/{id}, then query this endpoint with that page_id. Match to RSVPs by user_id. With group_by=referred_by_user the response becomes a referral leaderboard instead of submission rows: one row per referrer ({referred_by_user_id, count, user: {id, first_name, last_name}}), ordered by submission count descending, honoring the same filters.