Ignore Celery task results

Since we don't use the results of our Celery tasks (all of them return
None implicitly), it's prudent to set the ignore_result flag, for a
potential performance improvement. See the Celery docs for details [1].

We could do this with the global CELERY_IGNORE_RESULT setting, but it
offers more flexibility if we want to use task results in the future to
set it on a per-task basis.

[1]: https://docs.celeryq.dev/en/stable/userguide/tasks.html#ignore-results-you-don-t-want

bookwyrm/emailing.py

@@ -75,7 +75,7 @@ def format_email(email_name, data):
     return (subject, html_content, text_content)
 
 
-@app.task(queue=HIGH)
+@app.task(queue=HIGH, ignore_result=True)
 def send_email(recipient, subject, html_content, text_content):
     """use a task to send the email"""
     email = EmailMultiAlternatives(