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 have tried playing around with that but it doesn't seem to work. It seems there is an issue with the create form. I updated my code to this (which I think it what you were suggesting:

```
     def __init__(self, *args, **kwargs):
        if kwargs['instance']:
            initial = kwargs.get('initial', {})
            initial['keywords'] = kwargs['instance'].keywords_list
            kwargs['initial'] = initial
            task_id = kwargs.pop('task_id', 0)
            super(TaskForm, self).__init__(*args, **kwargs)
            self.fields['next_task'].queryset = Task.objects.filter(
                Task.is_available_filter()).order_by('name').exclude(id=task_id)
```

which works for the edit form, but for the create form I'm getting an error:

```
AttributeError at /tasks/create/
'TaskForm' object has no attribute 'fields'
```

I think that may be the reason I was only running the code if `task_id` was in `kwargs`.