[useradmin] Allow initial users to access all the apps

After a fresh install, no groups exist and new users don't have access to any app until a group with the launch permissions is created.

Like in Hue 1, we should probably:

1. create automatically a new group 'default' with all the apps 'launch' permissions
2. the 'default_user_group' property should default to 'default'

The logic maybe could be put with 'default_user_group'.