Add configurable next_url for OpenEdX Hawthorn login/register redirects

[igobranco]

Summary

The OpenEdX Hawthorn API previously hardcoded the richie prefix in the
?next= query parameter used during login and register redirects
(e.g. ?next=richie/courses/...). This prefix is used by OpenEdX's Nginx
configuration to identify that the redirect target belongs to the Richie
application.

This PR makes that prefix configurable per LMS backend, allowing sites with
custom Nginx routing rules to set their own prefix.

This allows to have multiple Richie sites connected to the same Open edX LMS.
And the Open edX redirects to the right Richie site during login/register redirects.

Changes

Backend

• Added JS_NEXT_URL optional setting to RICHIE_LMS_BACKENDS configuration
  (defaulting to "richie" if not set)
• Updated FrontendContextProcessor.get_lms_context() to include next_url
  in the frontend context when JS_NEXT_URL is configured
• Added tests for the LMS context processor covering: no backends, missing
  JS_NEXT_URL, present JS_NEXT_URL, and mixed multi-backend scenarios

Frontend

• Added optional nextURL option to APIOptions type
• Added optional next_url field to LMSBackend and AuthenticationBackend
  types
• Updated OpenEdxHawthornApiInterface to use options?.nextURL
  (falling back to "richie") for login/register redirects
• Updated LMSHandler to pass next_url from the backend config through to
  OpenEdxHawthornApiInterface
• Added tests for default and custom next_url behavior in both the Hawthorn
  API and the LMS handler

How to configure

RICHIE_LMS_BACKENDS = [
    {
        "BASE_URL": "https://lms.example.com",
        "JS_BACKEND": "openedx-hawthorn",
        "JS_COURSE_REGEX": r"...",
        "JS_NEXT_URL": "richie-mysite",  # optional, defaults to "richie"
    }
]

[liamls]

I have noticed an issue which may be blocking : the cookie cutter template is not updated here and should be added next to the other settings so generated projects expose this option.