Replace the generic Docker-based s3-npm-publish tool with a JS script in the
client repo which is tailored more specifically to the needs of client
deployments:

 - Avoid creating the `hypothesis@X.Y` and `hypothesis@X` aliases which we
   never used.

 - Support uploading a copy of the entry point under a version-indepenent
   alias, regardless of whether this is a pre-release or not.

   This will enable creating a stable URL which points to the current QA
   version of the client.

 - Deploy the package directly from the current working directory. This saves
   the need to wait for publication to npm to complete, or the need to
   actually publish the package to npm at all in the case of the QA
   release.