Provide registration variable containing all attachment URLs

[sergei-maertens]

Closes #4745

Changes

• Added registration variable to the Objects API with all the attachment URLs
• Added test

It's not a defect, but The Hague will probably very much appreciate if we backport this, and it looks very easy to do so.

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 96.58%. Comparing base (e4fb7bb) to head (4a1133e).

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #4841 +/- ## ======================================= Coverage 96.58% 96.58% ======================================= Files 749 749 Lines 25531 25539 +8 Branches 3376 3377 +1 ======================================= + Hits 24659 24667 +8 Misses 608 608 Partials 264 264 ```

:umbrella: View full report in Codecov by Sentry.
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