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.
Another alternative could be to use `get_form_kwargs` [0] to pass the any arguments to the form.

[0] https://docs.djangoproject.com/en/1.7/ref/class-based-views/mixins-editing/#django.views.generic.edit.FormMixin.get_form_kwargs