Add an option to only let paragraphs be used once

Wonder after 3 years if still a desired feature?

If still desired please reopen.

This is still a feature we require - currently, applying this patch is a part of our install & update process

Will need a ckeditor5 approach

liam morland → made their first commit to this issue’s fork.

I have created a merge request with the patch in #2. It seems to work on 4.0.x.