Fix AddLockVersionToPolls migration (#10200)

db/migrate/20190306145741_add_lock_version_to_polls.rb

+require Rails.root.join('lib', 'mastodon', 'migration_helpers')
+
 class AddLockVersionToPolls < ActiveRecord::Migration[5.2]
-  def change
-    add_column :polls, :lock_version, :integer, null: false, default: 0
+  include Mastodon::MigrationHelpers
+
+  disable_ddl_transaction!
+
+  def up
+    safety_assured do
+      add_column_with_default(
+        :polls,
+        :lock_version,
+        :integer,
+        allow_null: false,
+        default: 0
+      )
+    end
+  end
+
+  def down
+    remove_column :polls, :lock_version
   end
 end