Make MFE base URLs site‑aware via Site Configuration

Context:

There’s a draft PR to read LEARNING_MICROFRONTEND_URL from Site Configuration (with a Django settings fallback) across learning URLs and email tasks. The goal is to enable per‑Site (multi‑tenant / multi‑stage) overrides without deploy‑time env changes. https://github.com/openedx/edx-platform/pull/37504

Problem statement

Today, many places in edx-platform read MFE base URLs directly from Django settings (e.g., settings.LEARNING_MICROFRONTEND_URL). That makes multi‑site setups brittle: changing the MFE host for one site often requires redeploying or introducing bespoke settings plumbing. The draft PR (#37504) addresses this for Learning links and emails; this RFC proposes generalizing the approach to all MFE base URLs.

Proposal

• Replace direct reads of settings.<SOME_MFE_URL> with:
  configuration_helpers.get_value("<SOME_MFE_URL>", settings.<SOME_MFE_URL>)
  so operators may set a per‑Site override in Site Configuration; otherwise we fall back to the existing Django setting.

Candidate keys:

• LEARNING_MICROFRONTEND_URL
• PROFILE_MICROFRONTEND_URL
• ORDER_HISTORY_MICROFRONTEND_URL
• AUTHN_MICROFRONTEND_URL
• EXAMS_DASHBOARD_MICROFRONTEND_URL
• CATALOG_MICROFRONTEND_URL
• COURSE_AUTHORING_MICROFRONTEND_URL
• LEARNER_HOME_MICROFRONTEND_URL
• ACCOUNT_MICROFRONTEND_URL
• WRITABLE_GRADEBOOK_URL
• ENTERPRISE_LEARNER_PORTAL_BASE_URL

Why this is viable

• Aligns with how we already expect MFEs to be deployed (multi‑site, different hosts); the README even lists the override setting names we lean on in dev.
• Operator ergonomics: Site admins can change a single site’s MFE host without touching global settings or rebuilt containers.
• Low risk: Read‑only lookups.

Start by reviewing draft PR #37504 and tracing the listed MFE base URL settings across learning URLs and email tasks. Compare those usages with the candidate keys in this RFC; done means the proposed Site Configuration override and Django-settings fallback are generalized consistently across the identified MFE URLs.