Allow cross application menu grouping, including custom app groups and links

[darklow]

1. Allow cross application menu grouping
2. Allow custom app groups with custom url
3. Allow custom link with custom url (just a link to any location)
4. Allow same model to be in multiple groups same time
5. Allow set permission codename for custom app/link
6. This feature won't affect breadcrumbs (location bar) it is just for menu organization.

# Suggestion
SUIT_CONFIG = {
    'MENU_ORDER': (
        ('sites', ),
        ('auth', ('user', 'group',
                  {'name': 'Registration', 'url': '/custom_location/'}
        )),
        (('Custom group', '', 'icon-fire'), ('crm.contact', 'auth.user')),
        (('Logout', 'admin:logout', 'icon-remove'),), # or '/admin/logout'
    )
}