Consider security of environment

JupyterHub makes a big deal about not allowing users to alter their own environment for their single-user server:
https://jupyterhub.readthedocs.io/en/latest/reference/websecurity.html

First, I was wondering if I was missing something, because altering their own single-user would only serve HTML to themselves, so should be safe, right? (Besides, users of the same cluster could be considered "semi-trusted" but we should at least be aware of the issues)

But, assuming we need to be worried about this, we can evaluate the different spawners. For example, while setting up SlurmSpawner, I saw that by default, if `--export=VAR,...` is used, it will simulate a login shell and get the user's *clean* environment (as in, like what you might get on a fresh login) and then use that as the base, which directly allows people to do arbitrary env vars. Many spawners probably start a shell, which could evaluate config files. Basically, there are many possibilities here.

I had some mitigations:
- Use full path of `jupyterhub-singleuser` and start Python with `-E -s`

We can't fix every spawner and every cluster's own environment. We can provide documentation for batchsystem maintainers to consider when writing their scripts. We could make some tests for others to know if they are affected.

What do you think?