1071 allow selection of download file internal structure

[juzen2003]

• Fixes #1071
• Were any Django, import pipeline, table_schema, or dictionary files modified? N
  • If YES:
  • Database used: opus3_test_210331
  • All Django tests pass: Y
  • FLAKE8 run on modified code: Y
• Were any JavaScript or CSS files modified? Y
  • If YES:
  • JSHINT run on all affected files: Y (cart.js)
  • Tested on Chrome, Firefox, Safari, Edge, and iOS: Y (No iOS) (Remember to test all browser sizes)
  • Tested for race conditions (with delayed API calls if applicable): NA

Description of changes:

• Add a "Hierarchical" checkbox below the "Download Options" in the cart pane.
  • If checkbox is selected, the downloaded zip file will have all files stored with hierarchy tree.
  • If checkbox is not selected, in the zip file:
    • files with the same basename + same logical path (duplicated) will only be stored once.
    • files with the same basename + different logical paths (not duplicated) will be stored within its own hierarchy tree.
• API tests for /api/download/.zip & /__cart/download.json are added.
• For local testing purpose, the default selected files (0 byte dummy files) for these opus ids are stored in Shared OPUS dropbox:
  • co-iss-n1460973661
  • co-iss-n1481265970
  • co-iss-n1462840881

Known problems: N/A