[play] Allow configuring which binderhub play.nteract.io connects to

[yuvipanda]

Right now it is hardcoded to mybinder.org. It would be great if we could configure this somehow. I'd like playground.staging.mybinder.org to point to staging.mybinder.org, for example!

[rgbkrk]

Since I know we've buried it away in a chat somewhere, I'll summarize what I can remember -- the app itself should read particular values from environment variables (similar to how commuter does it), rendering a custom page with those values populated.

[stale]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.

[stale]

This issue hasn't had any activity on it in the last 90 days. Unfortunately we don't get around to dealing with every issue that is opened. Instead of leaving issues open we're seeking to be transparent by closing issues that aren't being prioritized. If no other activity happens on this issue in one week, it will be closed.
It's more than likely that just by me posting about this, the maintainers will take a closer look at these long forgotten issues to help evaluate what to do next.
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