add download ZIP file option to compound objects page

[juliastone0729]

It could be useful to be able to download all of a compound object's child objects at once as a ZIP file, instead of having to download each of the child object files individually

[evanwill]

This can't be done client-side, so would have to be an option build into the rake tasks in CB-CSV--which might make sense. Have some option to run rake zip-download (or something) that bundles all objects and metadata, then add a download button option with the data downloads.