Disable wysiwyg fields before they are changed

This prevents non-modified wysiwyg fields from being submitted (unless the forceSubmit option is used).
As a result, fields reformatted by tinymce are not submitted unless they have been modified by the user - thus avoiding generating 'fake' history.

| Q                    | A
| -------------------- | ---
| Bug fix?             | Y
| New feature?         | N
| BC breaks?           | N
| CI currently passes? | ?
| Tests pass?          | Y
| Scenarios pass?      | Y
| Checkstyle issues?*  | N
| PMD issues?**        | N
| Changelog updated?   | N
| Fixed tickets        | PIM-2965, PIM-2968
| Doc PR               | ~