apps.core.models.assignment_group: Added missing_expected_delivery property.
apps.core.models.assignment_group: Rename can_be_given_another_chance to

should_ask_if_examiner_want_to_give_another_chance.
apps.core.models.assignment_group: Docs.
devilry_examiner...singlegroupoverview: Always able to add deadline and

non-electronic delivery. Fixes #538. Implements the add deadline
suggestion for #513.