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/docker-compose/notebook/letsencrypt-notebook.yml

@@ -18,7 +18,7 @@ services:
       USE_HTTPS: "yes"
       PASSWORD: ${PASSWORD}
     command: >
-      start-notebook.sh
+      start-notebook.py
       --ServerApp.certfile=/etc/letsencrypt/fullchain.pem
       --ServerApp.keyfile=/etc/letsencrypt/privkey.pem