Split a person's name into first_name and last_name fields

This change does not get rid of the `name` field, but rather creates new users through a new form with `first_name` and `last_name` fields that populate the existing `name` field (which is used in other models).

It's my sense that a full migration might need to be handled separately, but I'd be happy to take a look if that's needed for this PR.