Alternative to parsing an arbitrary package's version number, or
its `build-info.json`.

I think this only reason we needed this was to support passing any
job id to `--build`, instead of requiring the `process_artifacts` job.
And to do that you needed to use the workflows API endpoint, which
requires an API token.

But now that the `--commit` argument exists and automatically finds the
correct job id, we can just use that.

Uses download-experimental script and places the base artifacts into
a top-level folder.

Replaces the two separate sizebot jobs (one for each channel, stable and
experimental) with a single combined job that outputs size information
for all bundles in a single GitHub comment.

I didn't attempt to simplify the output at all, but we should. I think
what I would do is remove our custom Rollup sizes plugin, and read the
sizes from the filesystem instead. We would lose some information about
the build configuration used to generate each artifact, but that can be
inferred from the filepath. For example, the filepath
`fb-www/ReactDOM-dev.classic.js` already tells us everything we need to
know about the artifact. Leaving this for a follow up.

The download script will poll the CircleCI endpoint until the build job
is complete; it should also poll the GitHub status endpoint if the
build job hasn't been spawned yet.