Use AppConfig in instuctor_task app to ensure tasks are discovered

This fixes an issue where instructor_task Celery tasks were not being
auto-discovered, thus leaving them unregistered and unable to be
completed by our Celery workers.

TNL-7652

lms/djangoapps/instructor_task/__init__.py

@@ -0,0 +1,2 @@
+# pylint: disable=missing-module-docstring
+default_app_config = 'lms.djangoapps.instructor_task.apps.InstructorTaskConfig'

lms/djangoapps/instructor_task/apps.py

@@ -0,0 +1,16 @@
+"""
+Instructor Task Application Configuration
+"""
+
+from django.apps import AppConfig
+
+
+class InstructorTaskConfig(AppConfig):
+    """
+    Application Configuration for Instructor Task
+    """
+    name = u'lms.djangoapps.instructor_task'
+
+    def ready(self):
+        # noinspection PyUnresolvedReferences
+        from . import tasks  # pylint: disable=unused-import