Extract counters from accounts table to account_stats table (#9295)

db/migrate/20181116165755_create_account_stats.rb

@@ -0,0 +1,12 @@
+class CreateAccountStats < ActiveRecord::Migration[5.2]
+  def change
+    create_table :account_stats do |t|
+      t.belongs_to :account, null: false, foreign_key: { on_delete: :cascade }, index: { unique: true }
+      t.bigint :statuses_count, null: false, default: 0
+      t.bigint :following_count, null: false, default: 0
+      t.bigint :followers_count, null: false, default: 0
+
+      t.timestamps
+    end
+  end
+end