Enable server extensions as both Notebook/Jupyter Server extensions

[lresende]

The Elyra image was working fine in JupyterHub/Binder until recently, which seems to have introduced a regression where Jupyter Server extensions are not being recognized. This has been reported/workarounded in a few JupyterLab extensions running on Hub/Binder environments (e.g. jupyterlab/jupyterlab-git#863) and we are following the same approach, which we used to have in the past as well.

I also looked into Hub/Binder to see if I could find the culprit commit, but I am not very familiar with where things are handled in Binder and didn't have much success and decided to stop.

Fixes #1474

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[elyra-bot]

Thanks for making a pull request to Elyra!

To try out this branch on binder, follow this link:

[lresende]

Tested this in both binder and regular local docker env and both are working with these changes.

[kevin-bates]

Could you please add an explanation as to why we need duplicated configurations for NotebookApp? This version of Elyra only uses Jupyter Server.

[ajbozarth]

code looks good, it looks the same as when I did this on nbdime. I have not tested local though

[kevin-bates]

These changes look good - thanks for tackling this!