Draft: Fix/rapid cdn slash cleanup

The commit https://lab.nexedi.com/nexedi/slapos/-/commit/821e25303807a28b050206dc52bd9cee575965ca results with bad behavior for Theia with websocket, it means that test coverage is not enough for this change.

Depends on https://lab.nexedi.com/nexedi/slapos/-/merge_requests/1935