Dialog w/options for Downloading Collections

From @stevecassidy on #2023.

> Just wanted to comment on this. An important use case for us would be for the user to upload a collection of files, run some processing pipeline on them and download the result. We can preserve the element_identifier between processing steps, but then they can't download the resulting dataset. If they could, the filenames would have a useless (for us) prefix named for the dataset collection . Better would be to download this as a zip file with a directory structure, the directory could be the dataset name but the individual data files would be ..

Dialog could offer a preview of different zip/tar contents - allow options such as using name vs. identifier. I would think the identifier is fairly universally more useful - but there is a request for using the name in #1675. Dialog would also give an entry point in the GUI for poll from an offer final download link as things get zipped up in the background.

The issue names no files, tests, or entry points. Start by tracing the GUI collection-download flow and the background zipping and download-link behavior described here; done means a dialog can preview archive contents, choose names versus identifiers, and provide a final download link.

Written by the indexing model from the issue text.