Registering the HijackUserAdmin is optional

## Problem

```gherkin
Given I have a custom CustomUserAdmin class for my User model
And the class doesn't extend the HijackUserAdmin model
And I don't want to add the hijack button for the model in admin
And I have `hijack` in my list of INSTALLED_APPS
And I registered the CustomUserAdmin as the User model admin class
And the HijackUserAdmin is registered as the User model  admin class
Then my CustomUserAdmin registration is invalid
# None of my changes can be seen in the Admin panel
Since is in conflict with the other registration
```

### Solution

We can make the admin registration of `HijackUserAdmin` optional. The registration will be disabled by setting the `SHOW_HIJACKUSER_IN_ADMIN` constant to False, in the Django settings file.