Add signup form

apps/submissions/apps.py

@@ -3,3 +3,9 @@ from django.apps import AppConfig
 
 class SubmissionsConfig(AppConfig):
     name = "apps.submissions"
+
+    def ready(self) -> None:
+        # Imports the module for its `@receiver` side-effect (registers
+        # `unlink_stl_file_on_delete` with the `post_delete` signal).
+        # See `apps/submissions/signals.py` and plan.md §7.6.
+        from . import signals  # noqa: F401