Make task lists optional for archived tasks

[joshsmith]

Problem

When a Task has archived: true in its changeset, the TaskList association should be made optional.

• change order_task in Task.changeset to set_order and from that function:
  • add validate_required([:task_list_id]) here instead of the main changeset
  • call order_task if archived: false
  • set the order to NULL if archived: false
• add tests for the changesets