Perform webhook enabling/disabling in a separate worker (if necessary)

Calling Webhook#enable! or Webhook#disable! after sending a payload means that the webhook service must always write to the DB, and thus cannot make efficient use of DB replicas.

Instead, we should only write when we have to (see #331917 (closed)), and do the enabling/disabling in a separate worker, so that the webhook execution worker can be purely read-only.