🐛 [#4653] Add blocks dropdown to TinyMCE toolbar (2.7.x backport)

[stevenbal]

this was implemented in 103e511, but that commit also introduces extra changes that we do not want to backport

partially closes #4653

Changes

• Add blocks dropdown to TinyMCE toolbar

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 (c3ba281) to head (ccb51ee).
Report is 3 commits behind head on stable/2.7.x.

Additional details and impacted files

@@              Coverage Diff              @@
##           stable/2.7.x    #4667   +/-   ##
=============================================
  Coverage         96.56%   96.56%           
=============================================
  Files               719      719           
  Lines             24157    24157           
  Branches           2873     2873           
=============================================
  Hits              23328    23328           
  Misses              563      563           
  Partials            266      266           

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