Fix the asynchronous conversion polling

The polling REST endpoint always returns 404 while the conversion is running.

The pollingURL redirects to the result when the conversion is done.
=> That's finally some kind of an issue when using it for real in an app (the mobile application for instance): there is no way to not follow the redirect, so there is no way to know when the conversion is done without downloading the whole result.

• Make the polling URL works and returns 200 with a JSON response containing the status and the result URL.
• Change the polling behavior to not redirect when the conversion is done, but let the client get the conversion result whenever he wants.