Allow setting of caseworking on a user

This commit changes the form that the user sees when inviting or editing
another user, if the service has the ‘caseworking’ permission set.

This will allow creating a new type of user, one who only has the
`send_messages` permission, without the `view_activity` permission.

We are doing this because we think there are a number of services with a
lot of users who don’t need to see the dashboard, or the other team
members, and that we can make a simpler interface for these users.

app/notify_client/models.py

@@ -185,7 +185,9 @@ class InvitedUser(object):
         return set(self.permissions) > set(permissions)
 
     def has_permission_for_service(self, service_id, permission):
-        return self.status != 'cancelled' and self.service == service_id and permission in self.permissions
+        if self.status == 'cancelled' and permission != 'view_activity':
+            return False
+        return self.service == service_id and permission in self.permissions
 
     def __eq__(self, other):
         return ((self.id,