Change subscriptions should have *separate* feature switches for

• tracking subscriptions and
• using subscriptions for change propagation.

The "useLegacyChangesSubscription" switch is used to enable subscription tracking on the client. On the repo, this only indicates the presence of the wb_changes_subscription table. Another switch should control whether the dispatchChanges script will use sitelinks (wb_items_per_site) or the new subscription table for dispatching.

The idea is that we want to enable tracking first, so we can eyeball the database before starting to use the subscription data for change propagation.