Add per box settings (cf #41)

This PR also:

- fixes model mixins that was not inheriting from `models.Model`, so fields was not active
- fixes `form-fullpage.html` template that was overriding `title` block
- introduces a `user.public_fields` method, allowing to loop over user fields, even with a custom user model

If it's better, I can split this into other PRs. :)
There's a `TODO` here, do you want to create an issue for that, and put the issue number in this comment?