Add users to default organization on creation

- we need to associate all users with at least one organization.
- For now, we only have the default organization, so we'll use that.
  That logic could change in the future depending on changes to
  user registration logic.
- Placement as a callback and the potential drawbacks to that approach
  were considered. However, due to how users are created during
  our different types of registration, not all go through our
  creation service. It is more likely they are handled by
  a create/save operation in the `Devise::RegistrationsController`