Optimize Status#permitted_for 500x (account timeline) (#5373)

The main change of this PR is removing `order by visibility` hack.

This was introduced to force using of `index_statuses_on_account_id` instead of PK index, but it seems no longer needed probably due to `index_statuses_on_account_id_id`. Removing this avoids reading all rows, so really improves first fetching of the user who has lot of statuses.

I have also changed JOIN to IN + subquery, which slightly faster in most cases.

app/models/status.rb

@@ -220,9 +220,7 @@ class Status < ApplicationRecord
         # non-followers can see everything that isn't private/direct, but can see stuff they are mentioned in.
         visibility.push(:private) if account.following?(target_account)
 
-        joins("LEFT OUTER JOIN mentions ON statuses.id = mentions.status_id AND mentions.account_id = #{account.id}")
-          .where(arel_table[:visibility].in(visibility).or(Mention.arel_table[:id].not_eq(nil)))
-          .order(visibility: :desc)
+        where(visibility: visibility).or(where(id: account.mentions.select(:status_id)))
       end
     end