[#4519] Decrease horizontal space of form variable dropdowns

[vaszig]

Closes #4519

Changes

• Added class name to the VariableSelection in order to define max-inline-size for the form variable dropdowns

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

• 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 96.56%. Comparing base (5580da4) to head (7782053).
Report is 493 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4558   +/-   ##
=======================================
  Coverage   96.56%   96.56%           
=======================================
  Files         725      725           
  Lines       24131    24131           
  Branches     3175     3175           
=======================================
  Hits        23303    23303           
  Misses        569      569           
  Partials      259      259           

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[sergei-maertens]

will merge and backport tomorrow

[sergei-maertens]

Backports:

• stable/2.7.x: e575180
• stable/2.6.x: 70da5e5

2.5.x was not really affected by this problem