Support progressive rendering

[trungleduc]

Fix #634

User-facing changes

voila3.mp4

• New CLI option --progressive_rendering to activate the progressive rendering mode

voila ... --progressive_rendering=True

• The dashboard appears immediately, with placeholders filling the cell outputs. These outputs are updated as the kernel executes each cell.
• --progressive_rendering and --preheat_kernel are incompatible.

Code changes

In progressive rendering mode:

• New Websocket handler is added at /voila/execution to receive execution request from the frontend and return the cell outputs.
• nbconvert does not execute notebooks anymore. The frontend will render the dashboard immediately, then it sends an execution request to the above Websocket endpoint.
• On Websocket response, the frontend receives cell output and its index, then OutputArea widgets are created at the corresponding cell to render the outputs.
• execute_input messages are filtered

Backwards-incompatible changes

N/A

[github-actions]

👈 Launch a Binder on branch trungleduc/voila/voicify

[martinRenou]

I'm confused about the tree snapshots changes, do you why it shows these changes?

[trungleduc]

Yeah me too. Something may have changed in the file browser package? I will check tomorrow.

[jtpio]

Looks like the previous snapshot was supposed to be the JupyterLab file browser, since it's the default now?

[martinRenou]

I just pushed a couple of commits to disable the progressive rendering feature when jupyter-server 1 is installed.

Apart from the changes in the ui-tests that I'm confused about, that looks good to me!