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.
Hey @bobsilverberg,

This is what I tried [0]  and it seems to work fine for me for both create and edit. Other than that the initial implementation looks good to me. It's totally up to you whether you choose one way or another.  r+wc :beers: 

[0] https://github.com/akatsoulas/oneanddone/blob/test-task-id/oneanddone/tasks/forms.py#L59

where `pk` is the argument [1] from the view.

[1] https://github.com/akatsoulas/oneanddone/blob/test-task-id/oneanddone/tasks/views.py#L374