Read-only mode for shared documents

[davidbrochart]

Problem

I think that we will soon need a read-only mode for showing documents, especially in the case of RTC.
With authorization coming to jupyter-server, protecting the PUT /api/contents/{path} endpoint with a contents:read permission could work: a user could change a document, but they would not be able to save it.
In RTC however, this becomes impracticle. Indeed, this endpoint is not used anymore, because saving a document happens inside the YDocWebSocketHandler, and this handler is needed even for loading a document. What users see is not only themselves editing a document, but also everybody else. So the ability to edit a document should be addressed a its source, in the frond-end.

Proposed Solution

It looks like CodeMirror 5 has a readOnly option. Maybe this is already possible using JupyterLab settings?

Additional context

See jupyter-server/jupyter_server#948.