PXD-365 ⁃ add mechanism to download declarative configuration overrides

[frickjack]

We currently check-in per-commons configuration overrides in data-portal code. Let's change that to instead only commit a generic configuration, and load overrides from a json file that is maintained in a public S3 bucket (aliased as public.internal via Route53) that data-portal accesses through reverse-proxy to load something like:

• https://commons.domain/public/config/portal.json

We could similarly offload access to some other static files that are currently served from gdcapi - ex:

• https://commons.domain/public/config/dictionary.json
• https://commons.domain/public/config/schema.json