Refactor ContentType/Field Reference

[toastercup]

Right now, a ContentType and its Fields are tied together by the Field's id. If a superadministrator is manipulating the ContentType via CLI, this is rather inconvenient. Instead, we should refactor the implementation so that Fields are tied to a ContentType via their unique, indexed name_id field. Let's research how we can do this performantly. If we're unable to do so, we should remove the name_id field and continue to reference by id, but create uniqueness validation logic scoped to a ContentType that normalizes the Field's name.