Wire up the Apps Nav Tool Backend

[JamieB-gu]

The Apps Nav Tool created in #14815 is a React client-side app. Once it's hosted in the admin tool (#14840), we need it to be able to make requests to the backend to persist the data it creates. The data is currently managed by cross-platform-navigation¹ in S3 in the mobile account, so we'd need an endpoint in the frontend admin app that can write to this bucket.

Footnotes

1. https://github.com/guardian/cross-platform-navigation ↩