Implement post_save hook to mirror django.contrib.admin.models.LogEntry

[sergei-maertens]

From open-formulieren/open-forms#2352 it's clear that LogEntry records are cascade-deleted when the user is deleted.

To not risk losing audit trails (one of the main reasons this library is often used), we should ensure that all admin LogEntry records are copied to timeline logger - this can be done with a post_save signal. It's important that we then also serialize the user representation (str(user), if given) into the JSON field data so this can still be retrieved when the user account is deleted (by accident or malicious intent).

This behaviour should be opt-in through a setting: TIMELINE_MIRROR_ADMIN_LOGENTRY = False.

Tasks

• Set up post save signal
• Implement/support templates for Django's action types if needed https://github.com/django/django/blob/main/django/contrib/admin/models.py#L17
• See if we can leverage the change_message logic: https://github.com/django/django/blob/main/django/contrib/admin/models.py#L109
• Ensure our log records are linked to the same object (https://github.com/django/django/blob/main/django/contrib/admin/models.py#L66)
• Ensure our log records are not deleted when the user is deleted