Add general signal interface for default data maintenance

This change is primarily intended to allow models to declare their own
default data, without having to use data migrations. Data migrations
have the downside of being run only once, so e.g. `manage.py flush` will
delete default data wchich will never be restored. Also, I consider it
more natural todefine default data setup on model classes.

In order to achieve that, this adds a generic interface for apps to
register hooks called on several signals, by simply implementing methods
with the same name on their AppConfig instance.

aleksis/core/apps.py

-from django.apps import AppConfig, apps
-
-from constance.signals import config_updated
-
 from .signals import clean_scss
+from .util.apps import AppConfig
 
 
 class CoreConfig(AppConfig):
@@ -10,5 +7,7 @@ class CoreConfig(AppConfig):
     verbose_name = "AlekSIS — The Free School Information System"
 
     def ready(self) -> None:
+        super().ready()
+
+    def config_updated(self, *args, **kwargs) -> None:
         clean_scss()
-        config_updated.connect(clean_scss)

aleksis/core/util/apps.py

 from importlib import import_module
+from typing import Any, Optional
 
 import django.apps
+from django.db.models.signals import post_migrate, pre_migrate
+
+from constance.signals import config_updated
 
 
 class AppConfig(django.apps.AppConfig):
@@ -17,3 +21,64 @@ class AppConfig(django.apps.AppConfig):
         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)
+        config_updated.connect(self.config_updated)
+
+        # Getting an app ready means it should look at its config once
+        self.config_updated()
+
+    def config_updated(
+        self,
+        key: Optional[str] = "",
+        old_value: Optional[Any] = None,
+        new_value: Optional[Any] = None,
+        **kwargs
+    ) -> None:
+        """ Called on every app instance if a Constance config chagnes, and once on startup
+
+        By default, it does nothing.
+        """
+        pass
+
+    def pre_migrate(
+        self,
+        app_config: django.apps.AppConfig,
+        verbosity: int,
+        interactive: bool,
+        using: str,
+        plan: List[Tuple],
+        apps: django.apps.Apps,
+    ) -> None:
+        """ Called on every app instance before its models are migrated
+
+        By default, it does nothing.
+        """
+        pass
+
+    def post_migrate(
+        self,
+        app_config: django.apps.AppConfig,
+        verbosity: int,
+        interactive: bool,
+        using: str,
+        plan: List[Tuple],
+        apps: django.apps.Apps,
+    ) -> None:
+        """ Called on every app instance after its models have been migrated
+
+        By default, asks all models to do maintenance on their default data.
+        """
+        self._maintain_default_data()
+
+    def _maintain_default_data(self):
+        if not self.models_module:
+            # This app does not have any models, so bail out early
+            return
+
+        for model in self.get_models():
+            if hasattr(model, "maintain_default_data"):
+                # Method implemented by each model object; can be left out
+                model.maintain_default_data()