Show mailing list at registration option

Added an option to check if a mailing should appear in the registration form for new user. Since the registration is owned by module.user, the use of this will be done in another PR

![image](https://cloud.githubusercontent.com/assets/4157103/5211940/3f938762-75ba-11e4-8e8d-5dc5c15cd7d2.png)


<!---
@huboard:{"order":1.875,"milestone_order":5,"custom_state":""}
-->

no, new mailing lists have a negative id to differentiate them. Before they all had -1, now they all have a unique negative value