Use Account.local.sum(statuses_count) instead of Status.local.count (#4996)

It is faster.

app/presenters/instance_presenter.rb

@@ -21,7 +21,7 @@ class InstancePresenter
   end
 
   def status_count
-    Rails.cache.fetch('local_status_count') { Status.local.count }
+    Rails.cache.fetch('local_status_count') { Account.local.sum(:statuses_count) }
   end
 
   def domain_count