Determine how to handle non-writable files

[dlqqq]

Description

The FileLoader in jupyter_collaboration includes this check before saving a file:

            m = await ensure_async(
                self._contents_manager.get(
                    path, format=model["format"], type=model["type"], content=False
                )
            )
            # Skip saving if file is not writable
            if not m["writable"]:
                return None

@Zsailer @jzhang20133 Do we also need this check? If so, can we only do the check once when the YRoomFileAPI initializes, or should we check this every time before we save a file?

Reference: https://github.com/jupyterlab/jupyter-collaboration/blob/9059310bb7d6b2e27ab9b5dc5989fb4c332a4678/projects/jupyter-server-ydoc/jupyter_server_ydoc/loaders.py#L156-L158

[ellisonbg]

I believe we do need to respect this and that we should do the test on each write. I can tackle this one.