Migrate start-notebook & start-singleuser to python (#2006)

* Migrate start-notebook.sh to bash

Based on

> Stop using bash, haha 👍

from https://github.com/jupyter/docker-stacks/issues/1532.

If there's more apetite for this, I'll try to migrate
`start.sh` and `start-singleuser.sh` as well - I think they should
all be merged together. We can remove the `.sh` suffixes for
accuracy, and keep symlinks in so old config still works. Since
the shebang is what is used to launch the correct interpreter,
the `.sh` doesn't matter.

Will help fix https://github.com/jupyter/docker-stacks/issues/1532,
as I believe all those things are going to be easier to do from
python than bash

* Rename start-notebook.sh to start-notebook

* Cleanup start-notebook a little

* Fix typo

* Migrate start-singleuser as well

* Remove unused import

* Run symlink commands as root

* Combine repetitive RUN commands

* Remove multiple args to env

-u can not be set by shebang, we must set the env var
instead

* Fix conditional inversion

Co-authored-by: Ayaz Salikhov <mathbunnyru@users.noreply.github.com>

* Fix how start-singleuser is exec'd

* Actually call jupyterhub-singleuser in start-singleuser

* Pass through any additional args we get

* Put .py suffix on the start-* scripts

* Add .sh shims for the start-* scripts

* Document start-notebook.sh and start-singleuser.sh

* Partially test start-notebook.sh

* Reflow warning docs

Co-authored-by: Ayaz Salikhov <mathbunnyru@users.noreply.github.com>

---------

Co-authored-by: Ayaz Salikhov <mathbunnyru@users.noreply.github.com>

examples/source-to-image/README.md

@@ -117,7 +117,7 @@ with the extra system packages, and then use that image with the S2I build to co
 The `run` script in this directory is very simple and just runs the notebook application.
 
 ```bash
-exec start-notebook.sh "$@"
+exec start-notebook.py "$@"
 ```
 
 ## Integration with OpenShift

examples/source-to-image/run

@@ -2,4 +2,4 @@
 
 # Start up the notebook instance.
 
-exec start-notebook.sh "$@"
+exec start-notebook.py "$@"

examples/source-to-image/templates.json

@@ -274,7 +274,7 @@
                     "name": "jupyter-notebook",
                     "image": "${APPLICATION_NAME}:latest",
                     "command": [
-                      "start-notebook.sh",
+                      "start-notebook.py",
                       "--config=/etc/jupyter/openshift/jupyter_server_config.py",
                       "--no-browser",
                       "--ip=0.0.0.0"