Integrate CKEditor 4 LTS

With CKEditor 4 reaching EOL, we will update all websites to CKEditor 4 LTS.

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Validations

• Make sure previous behavior is kept
• Extensions work as expected on clones
• Were the extensions enabled by default on new instances?

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Actions:

The LTS version comes with a license key that must be set to all instances.

The license should be kept secret, and as such the path /admin/config/ckeditor-lts/settings should be inaccessible to users, and key injection should be done outside the image code-base to avoid exposing it.

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Relevant MRs:

Cern-Drupal-Distribution MR: !251 (merged)