Move signal registering to AppConfig's ready from startup.py

common/djangoapps/course_modes/apps.py

+
+from django.apps import AppConfig
+
+
+class CourseModesConfig(AppConfig):
+    name = 'course_modes'
+    verbose_name = "Course Modes"
+
+    def ready(self):
+        import course_modes.signals  # pylint: disable=unused-import

common/djangoapps/course_modes/startup.py

-"""
-Setup the signals on startup.
-"""
-import course_modes.signals  # pylint: disable=unused-import