Fixed user save-signal issue with Haalcentraal BRP retrieval logging

[Bartvaderkin]

This was kind of gnarly:

1. We can't use pre_save signal because we need a saved user
2. Calling save() from either a pre_save or post_save handler creates a signal loop

Solution: use post_save so we have a saved user, and break the loop.

Downside is this will trigger any other post_save handler multiple times, which is a vote to centralize signal based user update stuff. Most of it is currently user_logged_in so not an issue.

Note this PR targets the open Django 4.x branch #1024

[pi-sigma]

Nice idea with checking/setting the flag in on_bsn_change! Strange how this could pass with Django 3.2 and only fail with the upgrade, but this seems like the right behavior.