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. :)
That is one ugly way to do it.

Have you thought about setting the `DJANGO_SETTINGS_MODULE` env variable? That would be much more useful than setting `IDEASBOX_ID` and then use that to come up with the django settings module name, what do you think?