Fix bug with collapsed metadata fieldset being hidden in JSON dump plugin

[viktorvanwijk]

Closes #5088

Changes

Ensure overflow: visible is set for the metadata variables fieldset in the JSON Dump plugin

Checklist

Check off the items that are completed or not relevant.

• Impact on features

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

• Release management

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

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

  • Ran ./bin/makemessages_js.sh
  • Ran ./bin/compilemessages_js.sh

• Dockerfile/scripts

  • Updated the Dockerfile with the necessary scripts from the ./bin folder

• Commit hygiene

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

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 96.74%. Comparing base (a1a1b00) to head (c9e42ed).
Report is 2 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5091   +/-   ##
=======================================
  Coverage   96.74%   96.74%           
=======================================
  Files         771      771           
  Lines       26636    26636           
  Branches     3467     3467           
=======================================
  Hits        25770    25770           
  Misses        605      605           
  Partials      261      261           

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