Sanitize `RequestPanel` data behind a setting

[tim-schilling]

We should sanitize the RequestPanel stats behind a setting. This would prevent us from storing passwords from monitored /login/ requests and other sensitive data. While this isn't a major concern while the toolbar runs in memory, when we start storing data this becomes a big concern.

This is somewhat related to #2048 (comment). See https://github.com/django/django/blob/main/django/views/debug.py#L115-L130 for an example on how to do this.