Add tooltips to flat zip file structure checkbox

[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? N
  • If YES:
  • JSHINT run on all affected files: NA
  • Tested on Chrome, Firefox, Safari, Edge, and iOS: NA (Remember to test all browser sizes)
  • Tested for race conditions (with delayed API calls if applicable): NA

Description of changes: Add tooltips to flat zip file structure checkbox in cart download pane.

Known problems: NA