Use @app.task instead of @shared_task

Previously, `@shared_task` was preferred by Resonant, but I realized
that `@app.task` will be able to provide better typechecking.
`@shared_task` is essential only for reusable apps (and `core` isn't
reusable). I'll make an upstream change to the Resonant
cookiecutter to prefer `@app.task` too.

Author: brianhelba

bats_ai/core/tasks/periodic.py

@@ -1,10 +1,10 @@
-from celery import shared_task
 from django.utils import timezone
 
+from bats_ai.celery import app
 from bats_ai.core.models import ExportedAnnotationFile
 
 
-@shared_task
+@app.task
 def delete_expired_exported_files():
     now = timezone.now()
     expired_files = ExportedAnnotationFile.objects.filter(expires_at__lt=now)