Serve local client-config.json if it exists

[matthew-white]

Now that we request client-config.json, I've been wondering whether it'd be possible to remove npm run dev:oidc. That command sets a single option in the config (oidcEnabled), but there are several other options in the config that a developer might want to set. The idea of this PR is that a developer can create a local client-config.json to match whatever they're working on, and those options won't show up in version control. Modifying src/config.js would have much the same effect, but it has the downside that it shows up in version control.

Related: #1034

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• run npm run test and npm run lint and confirmed all checks still pass OR confirm CircleCI build passes
• verified that any code or assets from external sources are properly credited in comments or that everything is internally sourced

[matthew-white]

We don't call add_header in this location, so I don't think we need to include this file. Locally, I can see that the expected headers are coming through.

[alxndrsn]

Should this change be a separate PR? It seems to be unrelated to the aim of this PR.

[alxndrsn]

I would keep this line, as it doesn't do any harm, and keeps this file closer to central's files/nginx/odk.conf.template, and therefore easier to compare.

We could go further in mirroring central's files/nginx/odk.conf.template by adding:

add_header Cache-Control no-cache;

[alxndrsn]

I like this approach because it's more consistent with how production behaves.