This commit introduces a few changes as a result of
customer issues with finding why a topic was relisted.
In one case, if a user edited the OP of a topic that was
unlisted and hidden because of too many flags, the topic
would get relisted by directly changing topic.visible,
instead of going via TopicStatusUpdater.
To improve tracking we:
* Introduce a visibility_reason_id to topic which functions
in a similar way to hidden_reason_id on post, this column is
set from the various places we change topic visibility
* Fix Post#unhide! which was directly modifying topic.visible,
instead we use TopicStatusUpdater which sets visibility_reason_id
and also makes a small action post
* Show the reason topic visibility changed when hovering the
unlisted icon in topic status on topic titles
