Creates a Goal for a site.

If the created goal is a revenue goal, it sets site.updated_at to be refreshed by the sites cache, as revenue goals are used during ingestion.

If a goal belongs to funnel(s), we need to inspect their number of steps.

If it exceeds the minimum allowed (defined via Plausible.Funnel.min_steps/0), the funnel will be reduced (i.e. a step associated with the goal to be deleted is removed), so that the minimum number of steps is preserved. This is done implicitly, by postgres, as per on_delete: :delete_all.

Otherwise, for associated funnel(s) consisting of minimum number steps only, funnel record(s) are removed completely along with the targeted goal.