Use autodiscover code of Django for checks and model extensions

Apart from not duplicating code, this allows us to move all loading to the core's ready() method, which ensures the code runs before thirs-party apps rely on it.

aleksis/core/util/apps.py

-from importlib import import_module
 from typing import Any, List, Optional, Sequence, Tuple
 
 import django.apps
@@ -19,15 +18,6 @@ class AppConfig(django.apps.AppConfig):
     def ready(self):
         super().ready()
 
-        # Run model extension code
-        try:
-            import_module(
-                ".".join(self.__class__.__module__.split(".")[:-1] + ["model_extensions"])
-            )
-        except ImportError:
-            # ImportErrors are non-fatal because model extensions are optional.
-            pass
-
         # Register default listeners
         pre_migrate.connect(self.pre_migrate, sender=self)
         post_migrate.connect(self.post_migrate, sender=self)
@@ -38,13 +28,6 @@ class AppConfig(django.apps.AppConfig):
         # Getting an app ready means it should look at its config once
         self.preference_updated(self)
 
-        # Register system checks of this app
-        try:
-            import_module(".".join(self.__class__.__module__.split(".")[:-1] + ["checks"]))
-        except ImportError:
-            # ImportErrors are non-fatal because checks are optional.
-            pass
-
     @classmethod
     def get_name(cls):
         """Get name of application package."""