Move the frontend into its own package

[rgaiacs]

In #1856, @yuvipanda wrote

Use react-router for URL parsing. This makes the BinderHub UI a SPA, which may be split into its own package separately in the future if so desired.

@rgaiacs and @arnim were talking and jupyterhub/mybinder.org-deploy#3264 will benefit of move the frontend into its own package to allow "embed" the frontend in a third-party website.

sequenceDiagram
    actor User
    participant browser
    participant gesis as lorem.ipsum.com
    participant binderhub as gesis.mybinder.org
    participant jupyterhub as hub.gesis.mybinder.org
    User->>browser: Type URL
    browser->>gesis: Make request
    gesis->>browser: Send response
    browser->>browser: Render form
    User->>browser: Fill form
    browser->>binderhub: Make build request
    binderhub->>binderhub: Build container
    binderhub->>browser: Inform of successful build
    browser->>jupyterhub: Make launch request
    jupyterhub->>jupyterhub: Start container
    jupyterhub->>browser: Inform of running container
    browser->>browser: Redirect to Jupyter Lab interface
    User->>browser: Python code
    browser->>jupyterhub: Run Python code

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