[MRG]: badge_base_url can be a function that gets the parameter from the HTTP RequestHandler

[manics]

This allows the badge_base_url setting and therefore the BINDER_LAUNCH_HOST environment variable to be set from a HTTP request, and is intended to allow a BinderHub to have different values of the variable depending on the original source of the request.

For example, Gesis currently has a mix of https://mybinder.org and https://notebooks.gesis.org/binder in different places. By changing badge_base_url to check for a parameter in the request it should be possible to have the URLs match consistently depending on whether the user was redirected from mybinder (e.g. the redirector could set a query parameter binder_launch_host=https://mybinder.org/) or directly (no parameter, default behaviour).

For example:

    def badge_base_url(handler):
        launch_host = (
            handler.request.headers.get('X-Binder-Launch-Host', '') or
            handler.get_argument('binder_launch_host', ''))
        print('launch_host', launch_host)
        if launch_host and not launch_host.endswith('/'):
            launch_host += '/'
        if launch_host == 'https://mybinder.org/':
            return launch_host
        return ''
    c.BinderHub.badge_base_url = badge_base_url

See discussion starting from jupyterhub/mybinder.org-deploy#1284 (comment)

I've tested this locally using curl:
curl http://binder-test-host/build/gh/manics/jupyter-offlinenotebook/7ba3520553c414d0b55e80b96c0f6df85f903078?binder_launch_host=https://mybinder.org
this will spit out a URL and token that can be copied into a browser.

To work with the mybinder federation this will require the parameter to be passed in proxy requests made by the redirector:

• https://github.com/jupyterhub/mybinder.org-deploy/blob/f92e921b031876ad7dd9f5ac05b33eba793c12a6/images/federation-redirect/app.py#L117
• https://github.com/jupyterhub/mybinder.org-deploy/blob/f92e921b031876ad7dd9f5ac05b33eba793c12a6/images/federation-redirect/app.py#L166