This commit introduce the concept of an user flag, this flag can be
defined in many places:
* globally trough a setting named A2_USER_<FLAG>
* on the user object itself if there is a property user.<flag> which is
not None
* by any hook returning a not None result and named a2_hook_user_<flag>
* for all users of an OU if the ou.<flag> is not None
It should contain the slug of the service from which the user was coming before
requiring login or registration. It's passed to the 'login' and 'registration'
event hooks.