user: Accommodate Django 3.1 change for model choice iteration

• Before Django 3.1, iterating the .choices for a field would yield (id, label) tuples directly suitable for use with ChoiceFields. From Django 3.1, id is an instance of ModelChoiceIteratorValue which helps to easily find the model instance. In most cases, using the proxy works, but in our case, the value is being hashed. Access the actual value of the field from the object to avoid this issue.

• Cleanup widget for disabling individual checkboxes in a group

• When a form is submitted, 'disabled' input field is omitted by the browser irrespective of its value. So, the last admin user, automatically add the 'admin' group to form values.

Tests:

• On Django 2.2 and Django 3.2 access the user edit page. The form should render as before the change without errors.

• Test with the current user as the last admin user. The 'admin' checkbox should be read-only.

• Test with the current user not as the last admin user. The 'admin' checkbox should not be read-only.

• Add/remove non-admin groups and save the current/different user.

• Access the user edit page as non-admin user, the groups should be disabled.

• Give/take admin permission to/from a user other than current user.

• Take admin permission from current user.

Signed-off-by: Sunil Mohan Adapa sunil@medhas.org