Add JSON response from web app when main page is requested with Content-Type: application/json

[ml-evs]

This would be very helpful for resolving the underlying API URL for users of the Python API. They may only "know" about e.g., datalab.odbx.science but the API will need to be passed api.odbx.science.

The web app should therefore have a JSON version of its root page that simply returns e.g., {"api_url": "<datalab_api_url>"} (and potentially some other metadata, like the description/owner of the datalab).

[ml-evs]

As discussed, probably want to hack the webapp/public/index.html to respond with some kind of structured response that the Python API can use to redirect when requested with HTTP header Content-Type: application/json

[ml-evs]

This turned out to be impossible -- potential workaround: add a meta field to HTML header that can be parsed by the Python.

<meta name="x-datalab-api-url" content="${{ VUE_APP_API_URL }}">