This should make the pages slightly quicker to load, because Redis will
return the JSON string faster than the API.
The only change that can happen to a broadcast which doesn’t go through
the admin app is a broadcast ending at its scheduled time. So this could
result in a cached broadcast having a status of `broadcasting` when it
had in fact finished. We already account for this here though:
b2b58ec044/app/models/broadcast_message.py (L89-L94)
