forbiddenCharacters data category - related to [ACTIOn-189]

Hi everyone,

Please find attached a new proposal for the Forbidden Characters data category.

Most of it is the same as Michael's initial text, I've just:

- re-worded a part that implied the data category was applying to translated content only.

- defined the regex syntax for the forbiddenCharacters value (it would be nice if we could have a pattern for it in the XML schema).

Cheers,
-yves