Allow services to decide if a request should be warned (`warn_user`)

[stsewd]

Currently, warning a user before redirecting it to another service is done by a global setting

django-mama-cas/mama_cas/forms.py

Lines 28 to 32 in 22e3b23

	if getattr(settings, 'MAMA_CAS_ALLOW_AUTH_WARN', False):
	self.fields['warn'] = forms.BooleanField(
	widget=forms.CheckboxInput(),
	label=_("Warn before automatic login to other services"),
	required=False)

Would be great if this behavior could be customized by a service backend

django-mama-cas/mama_cas/services/backends.py

Line 66 in 22e3b23

class SettingsBackend(object):

and similar to #112, have access to the current user, so the warning can be made taking into consideration the user and the service.