TinyMCE 8

[petschki]

• update tinymce to version 8.x and release it in new feature/major release of plone.staticresources
• make the licence option configurable via plone registry

[petschki]

done in #1516

see https://6.docs.plone.org/classic-ui/tinymce-customization.html#use-tinymce-with-a-commercial-license for setting up commercial licenses