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)


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I believe it should always be the `set` one, otherwise it's terribly confusing for the user. (I was confused myself at first)