use full dotted paths for all archivebox imports, add migrations and more fixes

archivebox/core/apps.py

@@ -5,8 +5,12 @@ from django.apps import AppConfig
 
 class CoreConfig(AppConfig):
     name = 'archivebox.core'
+    label = 'core'
 
     def ready(self):
         """Register the archivebox.core.admin_site as the main django admin site"""
         from archivebox.core.admin_site import register_admin_site
         register_admin_site()
+
+        # Import models to register state machines with the registry
+        from archivebox.core import models  # noqa: F401