Disabling shared drafts used to leave topics in an inconsistent state
where they were not displayed as shared drafts and thus there was no
way of publishing them. Moreover, they were accessible just to users
who have permissions to create shared drafts.
This commit adds another permission check that is used for most
operations and the old can_create_shared_draft? remains used just when
creating a new shared draft.
You can let non-staff users use shared drafts by modifying the `shared_drafts_min_trust_level` site setting. These users must have access to the shared draft category.
