Download Clinical Data from TSV API changed to POST

[joneubank]

Description of changes

Changes required to match API updates in: https://github.com/icgc-argo/platform-api/pull/663

clinical-data-tsv endpoint has changed to take POST request instead of default GET.

Type of Change

• [ ] Bug
• [ ] Dependency updates
• [x] Feature
• [ ] Refactoring
• [ ] Release candidate
• [ ] Styling
• [ ] Testing
• [ ] Other (please describe)

Checklist before requesting review

• Design (select one):
  • [ ] Matches design:
  • component sizes, spacing, and styles
  • font size, weight, colour
  • spelling has been double checked
  • [ ] No design provided, screenshot of changes included in PR
  • [x] No changes to UI
• Back-end (select one):
  • [ ] Back-end PRs merged, tested on develop, and linked in this PR
  • [ ] No back-end changes
• [ ] Manually tested changes
• [ ] Added copyrights to new files
• [ ] Connected ticket to PR