Deprecate/Remove `app` query parameter and the NBGITPULLER_APP default value

[consideRatio]

We have a app query parameter that is accepted in the /git-pull endpoint that influences the redirection path to unless urlpath is also set. A default value for this query parameter can be set via NBGITPULLER_APP.

I'm considering if we should remove it or attempt to deprecate it etc to reduce the complexity.

Background

@manics wrote

[...] there's an undocumented environment variable NBGITPULLER_APP

nbgitpuller/nbgitpuller/handlers.py

Line 149 in fdd54bd

app_env = os.getenv('NBGITPULLER_APP', default='notebook')

Do you know it's significance?

I replied

@manics it seem to have been added by me four years ago in #41 😮

I did some git history inspection:

• Ability to launch into JupyterLab #41 added NBGITPULLER_APP, a default value for an app parameter accepting notebook and lab.
• Documentation about it updated in Update README.md #48
• Documentation on constructing the url removed in favor of using a link generator in Add new link generator instructions #62

Overall, it seems that it does the single thing of "if set to lab", the web handler for /git-pull endpoint will default to prepending /lab/tree to a post git-pull redirection path. It won't affect gitpuller the CLI though.

I conclude that the https://jupyterhub.github.io/nbgitpuller/link only crafts links using urlpath directly though, and the NBGITPULLER_APP is a default value for a app query parameter, which only has an influence if urlpath isn't set.

I'd love to see this env var and the entire app query parameter removed to reduce complexity, but breaking existing links isn't fun. @yuvipanda do you have a suggestion with regards to something to do with the logic about having an app parameter and/or the NBGITPULLER_APP env which is the app parameters default value?

[yuvipanda]

Thank you for researching this thoroughly and opening this up, @consideRatio!

I think when external URLs that can live anywhere (emails, course websites, etc) are counted, we should try really hard to not break them. So my suggestion is to let these be, and perhaps leave a note saying 'do not add more things here, thanks'.

[consideRatio]

Thanks for the quick followup @yuvipanda! Sounds reasonable to me, what do you think @manics?

[manics]

Deprecating (but not removing) the app parameter makes sense.
What's the plan for the NBGITPULLER_APP env-var- remove it completely?

[yuvipanda]

Let's deprecate it (maybe a warning message if it is detected?) but leave it in?