Add button to quickly include all form variables in JSON dump plugin

[viktorvanwijk]

Closes #5066

Changes

Add button to quickly include all form variables in 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 96.72%. Comparing base (5afe627) to head (62bc105).
Report is 8 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #5070   +/-   ##
=======================================
  Coverage   96.72%   96.72%           
=======================================
  Files         770      770           
  Lines       26523    26524    +1     
  Branches     3454     3452    -2     
=======================================
+ Hits        25655    25656    +1     
  Misses        606      606           
  Partials      262      262           

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[viktorvanwijk]

I'm thinking now this might be better placed in Backend -> Admin styling?