allow downloading everything in a folder as a zip

closes: CNVS-14728
closes: CNVS-12658
refs: DES-213

test plan:
* in new files interface:
* click the "cloud download" button on an item
  (it's next to the publish and cog buttons)
* if it is a file, it should download immediately
* if it is a folder, it should show a progress
  indicator while it is preparing the download
  (delayed jobs need to be running)
* when the progress reaches 100%, the browser should
  download it just like any other file.

Change-Id: Icc4712b5b421e26343bcfbc6458459cfeedf471e
Reviewed-on: https://gerrit.instructure.com/40137
Tested-by: Jenkins <jenkins@instructure.com>
Reviewed-by: Jason Madsen <jmadsen@instructure.com>
Product-Review: Ryan Shaw <ryan@instructure.com>
QA-Review: Ryan Shaw <ryan@instructure.com>