Add "Download Zip" button for any directory inside a project

[nokome]

There is currently a download button for snapshots (which allows download of a zip of the contents of the snapshot):

and a download button for each file.

But there is not a download-as-zip for the project's working directory. A couple of users have indicated that this would be useful. It would provide similar functionality as GitHub's "Download ZIP" button:

We should also show a more button on folders in the Project's files list, and allow downloading the contents of a single folder within a project.

[mjohnson11]

It would also be great to be able to download subdirectories in the project as zips, if that is easy to implement alongside this!