Relocate binderhub's frontend UI from root folder

[consideRatio]

I'd like to compartmentalize things to help me understand the binderhub repo's code better. We now have a binderhub-client JS package to work against the binderhub REST API (/build) as a package to publish to NPM, and another one not to publish related to the binderhub's frontend.

The binderhub-client content is located clearly in js/packages/binderhub-client. I propose we try to locate the binderhub-ui parts to js/packages/binderhub-ui, or possibly binderhub/static/src or similar.

Overview of binderhub frontend js

• webpack.config.js, references:
  • binderhub/static/js/index.js, references:
    • src/badge.js
    • src/loading.js
    • src/path.js
    • ../index.css
• package.json
• babel.config.json
• .eslintrc.js

Help wanted

I don't know enough about this to get this done efficiently, but I think it would be really good to get done.

[manics]

Sounds fine to me. I'll add this to #1667