apps.core.models.assignment_group: Added ExaminerAssignmentGroupManager.
apps.core.models.deadline: Added query_successful_deliveries().

Also updated is_empty() and copy() to use query_successful_deliveries()
instead of deliveries. This prepares us for a solution like the one in #427.
apps.core.models.candidate: Added the displayname property.
devilry_examiner.views.singlegroupoverview: Basic layout for #488.
devilry_developer.testhelpers: Started on a new core testdata builder.
devilry_developer.testhelpers.corebuilder: Fixed some issues.
devilry_developer.testhelpers.corebuilder.UserBuilder: Default for ``email``.
devilry_developer.testhelpers.corebuilder: Use ``candidates`` instead of ``students``.
devilry_examiner.views.singlegroupoverview: Use the new corebuilder.

For tests.