Make Voila Dashboards work with Voila 0.3.5

[danlester]

Dashboards don't seem to work with Voila 0.3.5, probably due to new security in Voila.

Something like this might be needed: ideonate/cdsdashboards@148770e

[danlester]

I found that if Voilà was run with --preheat_kernel=False (the default, with True being a new feature in ~0.3) then jhsingle-native-proxy's ready-check fetch of the / URL would be connection refused, so we never believe Voilà is running. With --preheat_kernel=True I did not see the same problem.

This must be because the --preheat_kernel=False is not exactly backwards-compatible, although it is possible that increasing the timeout in Simpervisor would get around this (although maybe not for larger notebooks). Anyway, the simpler solution was to change the --ready-check-path=/voila/static/ passed to jhsingle-native-proxy in the Voilà case because that more immediately returns a valid response (albeit a 404).

I have changed this in cdsdashboards and released 0.6.1. When this is available on conda-forge I will update in QHub via a PR.