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>

images/base-notebook/start-singleuser.py

@@ -0,0 +1,23 @@
+#!/usr/bin/env python
+# Copyright (c) Jupyter Development Team.
+# Distributed under the terms of the Modified BSD License.
+import os
+import shlex
+import sys
+
+command = ["/usr/local/bin/start.sh", "jupyterhub-singleuser"]
+
+# set default ip to 0.0.0.0
+if "--ip=" not in os.environ.get("NOTEBOOK_ARGS", ""):
+    command.append("--ip=0.0.0.0")
+
+# Append any optional NOTEBOOK_ARGS we were passed in. This is supposed to be multiple args passed
+# on to the notebook command, so we split it correctly with shlex
+if "NOTEBOOK_ARGS" in os.environ:
+    command += shlex.split(os.environ["NOTEBOOK_ARGS"])
+
+# Pass any other args we have been passed through
+command += sys.argv[1:]
+
+# Execute the command!
+os.execvp(command[0], command)