✨ [#4969] Ensure map tile layers are loaded on deploy

Set up a post-migrate hook to load the map tile layers fixture.

src/openforms/config/apps.py

@@ -1,6 +1,16 @@
+from io import StringIO
+
 from django.apps import AppConfig
+from django.core.management import call_command
+from django.db.models.signals import post_migrate
+from django.dispatch import receiver
 
 
 class OpenFormsConfigConfig(AppConfig):
     name = "openforms.config"
     verbose_name = "Configuration"
+
+
+@receiver(post_migrate, dispatch_uid="load_default_map_tile_layers")
+def update_map_tile_layers(sender, **kwargs):
+    call_command("loaddata", "default_map_tile_layers", verbosity=0, stdout=StringIO())