Bug 1105491 - Users should not be able to make task it's own next task

Fix the Task Update form so that the current task does not appear in the "Next Task" dropdown

@glogiotatidis r?  I'm really not sure if this is the best way to do this. It seems a bit complex, but I couldn't think of a simpler way of giving the queryset access to the current task object.
I was thinking something similar to this. Thoughts?

```python
task_id = kwargs.pop('task_id')
super(TaskForm, self).__init__(*args, **kwargs)
self.fields['next_task'].queryset = Task.objects.exclude(pk=task_id)
..........
```