Prepare 2 User managers to exclude _system user by default

[slemrmartin]

This PR creates a AbstractDABUser who has 2 managers:

• User.objects....
• User.all_objects

this adds an ability to exclude some objects by default by apps using DAB, for example to exclude is_active=False or managed=True in the default queryset - without the need to rewrite queries in dependent apps

AAP-27023

[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

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