models: Update Person & UserProfile object whenever User is saved

models: Update Person & UserProfile object whenever User is saved

Currently, UserProfiles are only saved on creation of the User object,
before the first_name and last_name attributes are set. This means that
we fallback to using the User.username value as the new Person.name.

This change modifies User's post-save signal to always update the Person
object, both on creation and updates. This means we get the proper name
appearing on Person instances.

Signed-off-by: Jeremy Kerr <jk@ozlabs.org>