Allow modify the public trial state also in closed dossier states

:construction: 
To achieve this, we implement a special edit form, which only handles the public trial field.
The link is only visible on the overview tab, if the dossier is in a closed state and the user has "modify portal content" permission in a open dossier state. 

- [x] Implement special edit form (for document and mail)
- [x] Implement edit button on overview tab (document / mail)
- [x] Translations
- [x] One of the parents of a document, needs to be a dossier
- [ ] Hide public_trial on Repository and Dossier.
@maethu as discussed: Traverse up to the next dossier. I would suggest the following: Extract the [`find_parent_dossier()` method in `opengever.mail`](https://github.com/4teamwork/opengever.core/blob/master/opengever/mail/browser/extract_attachments.py#L273) to a function in `opengever.base.utils` and use that one.