Steps to reproduce

Note: This can only be reproduced on production, and only if the temporary Systems fixed is removed.

Example URL: https://archiveofourown.org/collections/COLLECTION_NAME/signups/summary

1. Create a gift exchange

2. Have five people sign up for the exchange

3. Wait for a Sign-up Summary to appear

What happens

The summary page says "The summary is being generated. Please try again in a few minutes." A summary may appear infrequently.

What should happen instead

The summary should always display.

Why does this happen

Anything that writes to a file and tries to share it will not work properly. We should move that shared state somewhere else, e.g. Memcached or Redis.