Use registration field order when using a registration extension form

1.  

   1. Description

On registration screen, make it possible to change the `REGISTRATION_FIELD_ORDER` when using a custom `REGISTRATION_EXTENSION_FORM`.

If you are managing an open edx installation probably you need to ask more information about your users on the registration form. For that you use a registration extension form. But when you configure that extension form you no longer can change the ordering of presentation of each field on the registration form. This pull request tries to fix that specific thing.

This change has impact for Developers and Operators that manage a custom open edx installation.

Example, with this configuration, we can
```
REGISTRATION_EXTRA_FIELDS:
city: required
confirm_email: hidden
country: required
gender: optional
goals: optional
honor_code: required
level_of_education: optional
mailing_address: hidden
year_of_birth: optional
terms_of_service: hidden
data_authorization: required # custom registration field
employment_situation: optional # custom registration field
allow_newsletter: optional # custom registration field

REGISTRATION_FIELD_ORDER:

• email

• name

• username

• password

• data_authorization # custom registration field

• honor_code

• city

• country

• allow_newsletter # custom registration field

• gender

• year_of_birth

• level_of_education

• employment_situation # custom registration field

• goals
  ```

*Required fields*:
*Optional fields*: