refs https://github.com/TryGhost/Arch/issues/87
- The Members Admin API and members.* webhooks were returning too many fields in the nested `newsletters` objects. There was no "allowlist" serializer for the newsletter object, which meant every time we add a new field to the database we would unintentionally return extra fields without a second thought.
- With this change only following fields will be returned with `members[x].newsletters[x]`:
'id',
'name',
'description',
'status'
