revert: restore the NAU account settings extension points

[efortish]

Reverts 2602e05, which removed the three extension points the account settings path called:

Extension point	Where	What it does
NAU_STUDENT_ACCOUNT_CONTEXT_EXTENSION	settings_views.account_settings_context	Renders the NAU fields on the account page
NAU_STUDENT_SERIALIZER_CONTEXT_EXTENSION	serializers.UserReadOnlySerializer	Returns their current values
NAU_STUDENT_ACCOUNT_PARTIAL_UPDATE	api.update_account_settings	Saves them

run_extension_point itself was never removed and is still live (lms/djangoapps/certificates/views/webview.py uses it), so this only puts the three call sites back.

Why

The account page is the only place a learner can fill or correct the NAU extended profile fields (nif, employment_situation, nuts, cae4) after registering.

ARTE (nau-technical#954) makes those fields part of the data NAU has to collect, and a course can block access until they are filled. Right now a learner hits that block and has nowhere to go: registration is the only path that writes to the model, and they already registered.

Why not the upstream replacement

Upstream replaced this pattern with PROFILE_EXTENSION_FORM (openedx/openedx-platform#37119), which landed after the Teak cut. We backported it in #47 and found two problems that make it unusable as it stands:

It writes every field on the extension model. That includes the ten cc_* fields populated from Autenticação Gov and the data_authorization privacy consent. get_registration_extension_form() serves both registration and account settings from the same setting, so one Meta.fields governs both paths and narrowing it for the account page also narrows registration, where data_authorization is required.

A partial update blanks everything else. The form is bound with the submitted data plus the existing row, so any field absent from the request validates as empty and is saved as empty. Sending only nuts through update_account_settings, on a real account:

antes    nif:123456789  cc_nif:987654321  cc_first_name:Kel  consent:True
despues  nif:None       cc_nif:None       cc_first_name:''   consent:False

Neither problem exists on this path. get_fields() in the plugin filters by the NAU_ACCOUNTS_CC_VISIBLE_FIELDS allowlist, so cc_* and the consent flag are never exposed unless someone lists them, and partial_update only setattrs the fields that actually arrive, so editing one field leaves the rest alone.

Not enough on its own

The plugin side was removed to match and has to come back too. That is in nau-openedx-extensions#159, which restores custom_registration_form/context_extender.py and the settings that point at it, and adds nuts and cae4 to the allowlist. Site configuration for NAU_ACCOUNTS_CC_VISIBLE_FIELDS goes in nau-tutor-configs#318.

With only this PR merged nothing changes: run_extension_point returns early when the setting is unset.

How to test

1. Deploy this together with the plugin branch from nau-openedx-extensions#159.
2. Set NAU_ACCOUNTS_CC_VISIBLE_FIELDS to ["employment_situation", "nif", "allow_newsletter", "nuts", "cae4"].
3. Open the account page as a learner. The five fields render, choice fields as dropdowns.
4. Change one of them and save. Check NauUserExtendedModel for that user: the edited field changed, the others kept their values, data_authorization untouched.
5. Remove nif from the allowlist and reload. The field disappears from the page and from the serializer payload.

What happens to #47

It stays open for now. If this lands, the PROFILE_EXTENSION_FORM backport is not needed for ARTE and we would rather close it than carry a fork divergence, but that is worth deciding together.