Polish backend for soft-required validation

[sergei-maertens]

Closes #4546

Changes

Tested this end-to-end with the SDK PR and noticed some more tweaks were needed.

• Make sure the HTML for the content is not treated as Django template
• Make sure the component is not displayed in summary page/PDF report/ registration data etc.
• Added tests
• Added translations for accessible list label/heading

Checklist

Check off the items that are completed or not relevant.

• Impact on features

  • [x] Checked copying a form
  • [x] Checked import/export of a form
  • [x] Config checks in the configuration overview admin page
  • [x] Problem detection in the admin email digest is handled

• Release management

  • [x] I have labelled the PR as "needs-backport" accordingly

• I have updated the translations assets (you do NOT need to provide translations)

  • [x] Ran ./bin/makemessages_js.sh
  • [x] Ran ./bin/compilemessages_js.sh

• Commit hygiene

  • [x] Commit messages refer to the relevant Github issue
  • [x] Commit messages explain the "why" of change, not the how

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 96.55%. Comparing base (7bae4ca) to head (fc2ffd7). Report is 6 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #4801 +/- ## ======================================= Coverage 96.55% 96.55% ======================================= Files 748 748 Lines 25399 25406 +7 Branches 3354 3357 +3 ======================================= + Hits 24525 24532 +7 Misses 609 609 Partials 265 265 ```

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[robinmolen]

Yeah lgtm 👍