Improve grouping of user creation errors in New Relic monitoring

Errors from the user creation API are not being grouped together properly in New Relic. This is because the error messages include [usernames and user IDs](https://github.com/hypothesis/h/blob/f1669fd89ce12aa7be53cbaaf161c4dbfa81e8b8/h/services/user_unique.py#L42) which vary with each message, but don't match any of NR's [automated rules](https://docs.newrelic.com/docs/errors-inbox/errors-inbox/#custom-groups) for ignoring values.

When a customer inadvertently triggered a large number of these errors, this made it difficult to tell NR to ignore them.

Slack thread: https://hypothes-is.slack.com/archives/C074BUPEG/p1716891448114809