add option for zip downloads

[jamiecook]

Our IT department prevents downloads of .py and .ipynb files.

This PR adds an option (documented) that allows for generating .py.zip and .ipynb.zip files instead.

[larsoner]

If this is a common enough use case -- that people's internet is restricted -- we should probably make all projects that use SG expose a .zip way of downloading files. In this sense it's a bit odd to make it a project-specific config value. We also already have too many config options, so it would be nice to avoid adding another.

So perhaps rather than a separate config option we could add a third download "entry" for all projects that use SG:

• Download Jupyter notebook: 10_overview.ipynb
• Download Python source code: 10_overview.py
• Download both in a zip archive: 10_overview.zip

That way people who can download directly still can, and people who have more restricted access can get what they need. I understand this is slightly less flexible in that people who have restricted access have to "download both" in all cases, but I think it's an acceptable tradeoff for those folks to have to (very easily!) delete the one small extra file they get once extracting, or more carefully just extract the one file that they want. The .zips should be tiny so no appreciable bandwidth/size problem should come from this either I think.

And if there are projects that hate this new .zip option, as long as we have it in part of its own CSS class, people can set visibility: hidden to make it go away.

[jamiecook]

Thats a good idea - I will modify and resubmit

[jamiecook]

@larsoner that is done, much smaller delta on this one now

[jamiecook]

I think this one just needs approval to run all the other CI workflows.

[larsoner]

Thanks @jamiecook !