Change account_id non-nullable in lists (#5979)

app/models/list.rb

@@ -4,7 +4,7 @@
 # Table name: lists
 #
 #  id         :integer          not null, primary key
-#  account_id :integer
+#  account_id :integer          not null
 #  title      :string           default(""), not null
 #  created_at :datetime         not null
 #  updated_at :datetime         not null