3576 existing crm customers

If a user already exists in the CRM and they then decide to register on the website. This is possible if for example they phone up.

If they signup with the same email address we don't create another CRM entry but instead associate the user with the existing CRM record.
This could be DRYed up thus:

```ruby
user.customer_id = if customer
  customer.id
else
  Registry::Repository[:customer].create(user)
end
user
```