This also limits the statuses returned by API, but pagination is not
implemented in Web API yet. I still expect it brings user experience
better than making a user wait to fetch all ancestor statuses and flooding
the column with them.

ancestor_statuses and descendant_statuses used to include the root status
itself, but the behavior is confusing because the root status is not
an ancestor nor descendant.

* Add a StatusFilter class to identify visibility of statuses by accounts

* Extract StatusThreadingConcern from Status

* Clarify purpose of checking for nil account