feat(release-notes): enhance unsubscribe functionality and user exper…

[viv-helix]

Summary

Enable token-based, login-free unsubscribe for Release Notes email notifications.

Previously, the unsubscribe page at /release-notes/unsubscribe required Studio login and called an authenticated GET endpoint. Users clicking the link from email were redirected to login, which blocked one-click unsubscribe.

This PR updates the flow so users can unsubscribe using a signed token from their email, without signing in.

Changes

• ReleaseNoteUnsubscribe reads ?token= from the URL and shows:
  • Confirmation UI when a token is present
  • Invalid-link error when the token is missing
• unsubscribeFromReleaseNoteEmails(token) sends an unauthenticated POST to /api/release_notes/v1/email/unsubscribe/ with { token } via getHttpClient() (replaces authenticated GET)
• index.jsx adds a custom auth handler so /release-notes/unsubscribe skips the login redirect while the rest of the MFE still requires authentication
• i18n adds invalid-link messages and updates error copy to reference the email link instead of signing in

User flow

1. User receives a Release Notes digest email with a link like:
   {COURSE_AUTHORING_MICROFRONTEND_URL}/release-notes/unsubscribe?token=<signed-token>
2. User opens the link — no login required
3. User confirms unsubscribe
4. Frontend POSTs the token to the backend; success or error is shown on the page

Backend dependency

Requires the corresponding edx-platform / release-notes backend change that:

• Generates per-user signed tokens in digest emails
• Exposes POST /api/release_notes/v1/email/unsubscribe/ with { "token": "..." }

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Test plan

• Open /release-notes/unsubscribe without a token query param → invalid link message is shown
• Open /release-notes/unsubscribe?token=<valid-token-from-email> while logged out → page loads without login redirect
• Click Unsubscribe with a valid token → success message; user is excluded from future digest emails
• Click Unsubscribe with an invalid/expired token → error message with retry option
• Confirm other Studio routes (e.g. /home, /release-notes) still require login

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Jira

• https://2u-internal.atlassian.net/browse/LP-815