Move status counters to separate table, count replies (#8104)

* Move status counters to separate table, count replies

* Migration to remove old counter columns from statuses table

* Fix schema file

db/migrate/20180812173710_copy_status_stats.rb

@@ -0,0 +1,19 @@
+class CopyStatusStats < ActiveRecord::Migration[5.2]
+  disable_ddl_transaction!
+
+  def up
+    safety_assured do
+      execute <<-SQL.squish
+        INSERT INTO status_stats (status_id, reblogs_count, favourites_count)
+        SELECT id, reblogs_count, favourites_count
+        FROM statuses
+        ON CONFLICT (status_id) DO UPDATE
+        SET reblogs_count = EXCLUDED.reblogs_count, favourites_count = EXCLUDED.favourites_count
+      SQL
+    end
+  end
+
+  def down
+    # Nothing
+  end
+end