Defining menu items can break for users without permissions

[nielsvanoch]

If you define the JET_SIDE_MENU_ITEMS setting with a list of models, for instance like this:

JET_SIDE_MENU_ITEMS = [
    {
        'label': 'foo',
        'items': [
            {
                	'label': 'bar',
                	'url': {'type': 'model', 'app_label': 'foo', 'model': 'bar'},
             },
        ],
    },
]

When a user that does not have any permissions to the foo.bar model tries to load the admin, they will trigger a KeyError here: https://github.com/assem-ch/django-jet-reboot/blob/master/jet/utils.py#L310, since the model won't be present in models.