Dynamic MARTOR_UPLOAD_URL

With current implementation, the upload url has to be set via settings.py. In case the upload url has to be changed dynamically in certain scenarios, controlling it via MartorWidget(upload_url="https://some-url") would make more sense ?

Start by tracing MARTOR_UPLOAD_URL from settings.py into MartorWidget, focusing on how the current upload URL is passed to the editor. The work is done when a widget-level upload_url can override the setting while existing configuration behavior remains available.

索引モデルが issue の本文から書いたものです。