devilry_examiner...add_deadline: Usable.

The UI makes it hard not to include a message as explained in #435.
devilry_examiner...add_deadline: Cleanup the view.
devilry_examiner...bulkviewbase: Added get_initial() just like Django FormView.
devilry_examiner...add_deadline: Require empty text when checking the option,

and support initial_text parameter to provide a default text.
devilry_examiner...add_deadline: Added the tools needed for giving another try.