Fixed #22295 -- Replaced permission check for displaying user-tools

Since has_permission() already checks if the user is staff and is active, I replaced {% if user.is_staff and user.is_active %} by {% if has_permission %} in base.html.
So, if someone creates a custom admin with a custom has_permission(), the user-tools will be displayed.
It's also backward compatible for someone who extends base.html and expects the user-tools to be hidden.
```
kwargs.update({
    'root_path': urlresolvers.reverse('admin:index'),
    'has_permission': admin.site.has_permission(self.request),
})
```