chore(staff): Activate staff middleware (#62890)

Starting to test this locally, so activating the middleware. This will
attach the staff object to all requests, although it doesn't do anything
as long as you don't include checks for staff (which we currently don't
do in prod).

src/sentry/conf/server.py

@@ -346,6 +346,7 @@ MIDDLEWARE: tuple[str, ...] = (
     "sentry.middleware.customer_domain.CustomerDomainMiddleware",
     "sentry.middleware.sudo.SudoMiddleware",
     "sentry.middleware.superuser.SuperuserMiddleware",
+    "sentry.middleware.staff.StaffMiddleware",
     "sentry.middleware.locale.SentryLocaleMiddleware",
     "sentry.middleware.ratelimit.RatelimitMiddleware",
     "django.contrib.messages.middleware.MessageMiddleware",