:bug: [#4653] Add blocks dropdown to TinyMCE toolbar (2.6.x backport)

[stevenbal]

this was implemented in https://github.com/open-formulieren/open-forms/commit/103e511979080910fb98cc9d6e97500faac45f91, 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

  • [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

[sergei-maertens]

Checked our "prod" environment which is on 2.6.14 and it's not broken there, so I'm declining this backport.