Add a mechanism to verify app builds during CI

[arbrandes]

Description

By design, npm run build currently does not work in app repositories (apps are supposed to be distributed "build-less"). However, we still want a way to verify that they can in fact be built as one of the steps in CI.

So, find a way to use either site.config.test.tsx or a new site.config.ci.tsx to use as a basis for a build step that's only supposed to be used during CI. The package.json script that invokes this should be called something other than "build" to avoid confusion: "npm run ci-build", for instance.

Notes

It's possible that most (or even all) changes will take place in individual app repos, but the process should still be documented in the migration howto in frontend-base itself, as well as becoming part of frontend-template-app.