This brings type-checking to the last place we fetch
data from Redis, with the exception of our APNs code
which is being replaced (with a Redis-free version,
thanks to improvements in Apple's APNs API) shortly.
Make it less likely that further development will break compatibility with
ZULIP_ADMINISTRATORs of the form "name <email>".
Note that the suggested value for this setting has been
'zulip-admin@example.com' for a while, so hopefully this commit causes no
change for most installations.