apps.core.models.delivery: Removed hard deadline validation.

See #434.
devilry_student.rest.add_delivery: Removed unused imports.
devilry_student.rest.add_delivery: Validate hard deadlines.

See #434.
devilry_student: Moved hard deadline error message from core.

See #434.
devilry_student.rest.add_delivery: Convert error messages to unicode.

Can not serialize gettext_lazy objects.
src/devilry_student/devilry_student/tests/rest/add_delivery.py
 * Added test for hard deadlines after porting to API check instead of
 core check.
src/devilry_student/devilry_student/tests/rest/add_delivery.py
       * Switched to enumerated deadline setting