NextJS: static assets should use the CDN

[eatyourgreens]

Package

app-project
app-content-pages

Is your feature request related to a problem? Please describe.

Static assets are being served direct from fe-project.zooniverse.org. See the list here:

https://www.webpagetest.org/result/211012_AiDc90_8b632dd922250dee0f45d20c722a5bd5/1/performance_optimization/

Describe the solution you'd like

Set up prefixes, in the Next config, for the NextJS apps, then use those prefixes to serve _next for each app via www.zooniverse.org/projects/_next and www.zooniverse.org/about/_next.

There's a couple of static images for projects, which should also be proxied:
https://github.com/zooniverse/front-end-monorepo/tree/master/packages/app-project/public

Static proxying will need to be set up for staging and production, and the asset prefixes for the apps will need to be updated.

Context

NextJS 9.5 added basePath, to allow for multiple apps running on a single domain. We can use that to set up both Next apps on www.zooniverse.org, which then allows Next's static assets to be served with CDN caching.

I think the order of operations here would be:

1. Add basePath to each app's config, and verify that it works.
2. Add rules for _next/ and public/ to our static setup. Add rules for production and staging NextJS apps static#253
3. Update the asset prefixes, in this repo, from fe-project and fe-content-pages subdomains to www.zooniverse.org (production) and frontend.preview.zooniverse.org (staging.) Use CDN domains for asset prefixes #2541

[eatyourgreens]

Reopened by #4225.

[goplayoutside3]

Closed by #4292