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.
Maybe you could use `self.fields['next_task'].queryset = your_queryset` after calling `super`. Not much of a change but should be more readable. Thoughts?