ju/ednx/JU-11: Fix custom registration

[mariajgrimaldi]

This PR adds the ability to customize the registration form using settings.

For example:

If we want to add the field Pet name we would have to do the following:

1. Create a definition dictionary for our custom field:

        {
            "errorMessages": {
                "required": "Add your pet name."
            },
            "instruction": "Your pet name",
            "label": "Pet name",
            "name": "pet_name",
            "restrictions": {
                "max_length": 10,
                "min_length": 3
            },
            "type": "text"
        }

2. Add that dictionary to EDNX_CUSTOM_REGISTRATION_FIELDS
3. Add name to extended_profile_fields
4. Add name to REGISTRATION_EXTRA_FIELDS with the value optional or required

    "REGISTRATION_EXTRA_FIELDS": {
        "city": "hidden",
        "confirm_email": "hidden",
        "country": "hidden",
        "gender": "optional",
        "goals": "optional",
        "honor_code": "required",
        "level_of_education": "optional",
        "mailing_address": "optional",
        "pet_name": "required",
        "specialty": "required",
        "terms_of_service": "hidden",
        "year_of_birth": "optional"
    }

5. Optional: if you want to specify: REGISTRATION_FIELD_ORDER