Project: new `api_retrieve()` classmethod

This is more direct than `webdriver_retrieve()`, with the usual
benefits of API methods: performance and stability

In this particular case, we were relying on the full session cookie
to be accepted in the API, which is not guaranteed to be acceptable
in the long run.

At this point `webdriver_retrieve()` is not called any more, but
we're not removing it right away.
2 jobs for topic/default/modernisation in 29 minutes and 55 seconds