Redone work items don't update previous work item property

[anehx]

If a work item with the status redo is being reopened by the workflow, the property previous_work_item is not properly updated which results in a wrong workflow tree.

We should update the previous_work_item on the reopened work item when we reopen it here:

caluma/caluma/caluma_workflow/utils.py

Lines 117 to 125 in 71be792

	if work_items_to_redo.exists():
	for work_item in work_items_to_redo:
	work_item.deadline = task.calculate_deadline()
	work_item.modified_by_user = user.username
	work_item.modified_by_group = user.username
	work_item.status = models.WorkItem.STATUS_READY
	work_item.save()
	work_items.append(work_item)
	continue