Build: Streamline frontend config

Background: Currently, in order to build the frontend workspace, one needs to create two files: index.html and firebase_config.ts. There are example files for each (index.example.html and firebase_config.example.ts). Minimally, one must copy the example files to build. The files themselves are .gitignored to prevent committing.

This design does prevent marginally sensitive values from being committed to the repo (such as the Firebase API key). However, it does not take full advantage of Webpack's ability to perform substitutions and inject environment variables.

Proposal:

1. Delete both *.example.* files.
2. Remove the exclusions from .gitignore.
3. Make both files depend on Webpack-injected fields (no hard coded secrets).
4. Check in the config-based files.
5. Update GitHub Actions (`ci.yaml') as needed to build using the streamlined procedure.
6. Update the Google Cloud Build (cloudbuild.yaml) as needed.
7. Update documentation on how to build and deploy based on the updated method.

It may make sense to create files like firebase_config.dev.json and firebase_config.prod.json, the latter of which would be .gitignored. Then, in the Webpack build, bring in these files' contents for the particular kind of build being performed.

See #996 for discussion on streamlining the frontend build process.

Start by reading the example files, .gitignore, ci.yaml, cloudbuild.yaml, and the existing Webpack configuration, then review discussion in #996. Map how frontend builds currently obtain index.html and firebase_config.ts, and update the workflow, Cloud Build configuration, and build/deploy documentation so the streamlined procedure works without the example files.