Add NGINX's unique request_id to Sentry logs

We might want to do this for our other apps as well:

NGINX can generate a unique request_id for each request. This shows up in the NGINX logs in Papertrail and it can also be added to Sentry issues. This will help with correlating Sentry issues with logs in Papertrail.

Suggested by @ndmanvar [here](https://github.com/hypothesis/via3/issues/27#issuecomment-577899535). There's also a [Sentry blog post about how they do it](https://blog.sentry.io/2019/01/31/using-nginx-sentry-trace-errors-logs).