Fixed #18681 -- GenericInlineModelAdmin.get_formset() no longer bypasses get_fieldsets().

Refs 23e1b59 which already fixed this issue for ModelAdmin and InlineModelAdmin.
Fixed LogEntry.get_admin_url() for non-existent models.

Regression introduced by [369b6fa]; refs #18169.
Fixed #18923 -- Corrected usage of sensitive_post_parameters in contrib.auth

Thanks Collin Anderson for the report.
Introduced ModelAdmin.get_fields() and refactored get_fieldsets() to use it.

Refs #18681.