Added Admin integration permissions for retrieving and managing newsletters

refs https://github.com/TryGhost/Toolbox/issues/238#issuecomment-1124024042

- Admin integrations require access to managing newsletters to prevent a regression in Zapier capability

core/server/web/api/canary/admin/middleware.js

@@ -31,6 +31,7 @@ const notImplemented = function (req, res, next) {
         members: ['GET', 'PUT', 'DELETE', 'POST'],
         tiers: ['GET', 'PUT', 'POST'],
         offers: ['GET', 'PUT', 'POST'],
+        newsletters: ['GET', 'PUT', 'POST'],
         config: ['GET'],
         schedules: ['PUT'],
         files: ['POST'],