Import models when django ready

Since we store model's instances in cache, this will help django figure out the correct model when unserializing cache data.

chat_box/apps.py

@@ -3,3 +3,6 @@ from django.apps import AppConfig
 
 class ChatBoxConfig(AppConfig):
     name = "chat_box"
+
+    def ready(self):
+        from . import models

judge/apps.py

@@ -12,7 +12,7 @@ class JudgeAppConfig(AppConfig):
         #          OPERATIONS MAY HAVE SIDE EFFECTS.
         #          DO NOT REMOVE THINKING THE IMPORT IS UNUSED.
         # noinspection PyUnresolvedReferences
-        from . import signals, jinja2  # noqa: F401, imported for side effects
+        from . import models, signals, jinja2  # noqa: F401, imported for side effects
 
         from django.contrib.flatpages.models import FlatPage
         from django.contrib.flatpages.admin import FlatPageAdmin