member_list action returns deleted users

[avdata99]

CKAN version
2.9.7

Describe the bug
The member_list action with 'object_type': 'user' returns all member users, including deleted ones
This was fixed with #3078 but relies on the user_delete action.
If I manually delete a user from the DB, this will fail.

Steps to reproduce
Trying to iterate all members, you will get toolkit.ObjectNotFound for deleted users

 users = toolkit.get_action('member_list')(context, {
            'id': container['id'],
            'object_type': 'user',
        })
       
for user in users:
     duser = toolkit.get_action('user_show')(context, {'id': user[0]})

Expected behavior
member_list action must return only active users

Possible fix
Add an extra DB query to filter only active users

[amercader]

To mimic the behaviour at the Python level introduced in #3078 (mark membership records as deleted when you delete a user via user_delete) we need to implement a foreign key with the proper CASCADE behaviour between the user and member tables.
Additionally we can add an extra check to the SQLAlchemy query in member_list that filters member records who don't have a record in the user table.
The same fix can probably be applied for datasets

[chetan-NEC]

@avdata99, @amercader I'm interested to work on this issue.