Bump ftw.mail to 2.0

This PR contains the changes needed to upgrade `ftw.mail` to version 2.0:

- Removed `IMailInAddress` behavior. It's not needed any more with `ftw.mail` 2.0, because that functionality is already included there 
- Provide an `IntIdResolver` adpater to generate Dossier Mail-In addresses. `ftw.mail` contains a default `UUIDResolver`, so we need to register our own adapter to deal with IntIds. 
- Removed `opengever.mail.interfaces.IMailSettings` registry interface. These settings have been migrated to `ftw.mail` itself.
- Hide standard `ftw.mail` mail-in viewlet. In OpenGever, we already display the mail address in the custom dossier byline.
- Update mail related tests.