use prod redis for staging and prod explicitly

This was already the case, but the staging environment was using the prod redis backend right now on crm-ext-01. This is actually useful: it allows us to test the site before it goes live.

I'm not sure it's a good idea, but it's the way things are setup now.

So let's make that explicit and also include staging in that allowlist of environments, instead of assuming "is_dev()" actually works (it really doesn't).

See tpo/web/donate#10