Make VoilaApp inherit from JupyterApp?

[maartenbreddels]

We now inherit from Application, should we inherit from JupyterApp (see discussion in #216) , or can jupyter_server help out here?

[SylvainCorlay]

I think that we should wait for the ExtensionApp system to be in jupyter_server and have voila be an ExtensionApp.

[vidartf]

Anything I can do to move this along? :)

[vidartf]

Xref jupyter-server/jupyter_server#48

[SylvainCorlay]

@vidartf I am fine with the current state of the PR. How about we merge it today. I just leave it open so that you have a chance to give it a look?

One thought that relates to voila:

• once we adopt this approach, we will make voila an ExtensionApp all the time. This will allow us to drop the VoilaConfiguration mixin and have all the configuration into VoilaApp.
• then jupyter voila will run a jupyter_server and with the start URL set to /voila.
• we will then probably want the voila command line to be an alias on jupyter voila?
  (but with an additional option passed to jupyter-voila when using the voila alias that disables all other jupyter_server extensions such as notebook, so that we don't serve the notebook or jupyterlab by accident, and give a means to execute arbitrary code.

[vidartf]

@SylvainCorlay Regarding your third point: Which bit is it that will end up loading the different server extensions? The ExtensionApp, or the contained ServerApp? Would it be possible for the extension app to have a different configuration set of server extensions? E.g. each extension app could choose either:

• To inherit all extensions from parent (jupyter serverextension enable), plus any extensions specifically enabled for it, minus any extensions specifically disabled for it (enabled/disabled by user).
• To not inherit any extensions from parent, but only have extensions specifically enabled for it (useful for voila).

[Zsailer]

See #270

[maartenbreddels]

Won't do, #270 is better