✨(backend) handle custom redirect URL while login

mozilla-django-oidc now supports a 'returnTo' parameter for redirecting
to a specificURL upon successful login. if not provided,
it defaults to the settings-defined URL.

This allows initiating the login flow from any views,
enhancing UX by returning users to their previous page.

The 'returnTo' naming can be discussed.

src/backend/meet/settings.py

@@ -344,6 +344,9 @@ class Base(Configuration):
     ALLOW_LOGOUT_GET_METHOD = values.BooleanValue(
         default=True, environ_name="ALLOW_LOGOUT_GET_METHOD", environ_prefix=None
     )
+    OIDC_REDIRECT_FIELD_NAME = values.Value(
+        "returnTo", environ_name="OIDC_REDIRECT_FIELD_NAME", environ_prefix=None
+    )
 
     # Video conference configuration
     LIVEKIT_CONFIGURATION = {