Fix null constraint violation in copy status stats migration (#8198)

db/migrate/20180812173710_copy_status_stats.rb

@@ -4,8 +4,8 @@ class CopyStatusStats < ActiveRecord::Migration[5.2]
   def up
     safety_assured do
       execute <<-SQL.squish
-        INSERT INTO status_stats (status_id, reblogs_count, favourites_count)
+        INSERT INTO status_stats (status_id, reblogs_count, favourites_count, created_at, updated_at)
-        SELECT id, reblogs_count, favourites_count
+        SELECT id, reblogs_count, favourites_count, created_at, updated_at
         FROM statuses
         ON CONFLICT (status_id) DO UPDATE
         SET reblogs_count = EXCLUDED.reblogs_count, favourites_count = EXCLUDED.favourites_count