Page comments

Enable comment on specific pages like /service/email/mailbox_details/test@example.org/.

• Users and admins can write 0-1 comments each.
• (optional) comments have an option "cascading" which makes /service/email/s comment visible at /service/email/mailbox_details/ etc.

Open Questions

• Is this going to carnivora and if that's the case, is it a new schema?
• What about i18n?