Improve the user experience of the form variable select/dropdown

[sergei-maertens]

Closes #4524 Closes #4675

I've cleaned up the commit history from the original PR

Changes

The variable dropdown now uses the react-select component. Allowing for searching within the options

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 96.56%. Comparing base (d0a504f) to head (72859d8). Report is 13 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #4730 +/- ## ======================================= Coverage 96.56% 96.56% ======================================= Files 746 746 Lines 25191 25191 Branches 3313 3313 ======================================= Hits 24325 24325 Misses 602 602 Partials 264 264 ```

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