Change account_id non-nullable in lists (#5979)

db/migrate/20171201000000_change_account_id_nonnullable_in_lists.rb

@@ -0,0 +1,7 @@
+require Rails.root.join('lib', 'mastodon', 'migration_helpers')
+
+class ChangeAccountIdNonnullableInLists < ActiveRecord::Migration[5.1]
+  def change
+    change_column_null :lists, :account_id, false
+  end
+end