devilry_subjectadmin: Organized views.py into a module.
devilry_developer...soupselect: Added prettyhtml().
devilry_subjectadmin: Added assignment_breadcrumb include template.
devilry_examiner...add_deadline: Improved the datetime widget.
devilry_subjectadmin: Usable editing view for assignment.

Needed for #520.
apps.core.assignment: Better help texts.
devilry_subjectadmin...assignment_update: Cleanup.
apps.core.models.assignment: Set choices for deadline_handling.
devilry_subjectadmin...test_assignment_update: Updated for delivery_types choices.
apps.core.models.custom_db_fields: Spellfix.
devilry_subjectadmin...assignment_update: Permission checks.
devilry_subjectadmin...assignment: All edit links go the the new assignment update view.

See #520.
devilry_subjectadmin: Built JS.