Updates warning message on double file open

[JasonWeill]

If the same file is opened twice in the same JupyterLab session, in two different contexts, we had previously displayed a message reading:

Two collaborative sessions are accessing the file (name) simultaneously. Opening a document with multiple views simultaneously is not supported. Please close one view; otherwise, you might lose some of your progress.

This change rewrites the message to:

You have opened the file (name) with two separate views. This is not supported. Please close one view; otherwise, some of your edits may not be saved properly.

This is intended to be clearer, and is intended to fix or mitigate #371. "Two collaborative sessions are accessing" is misleading; two different sessions can, by design, edit the same file. This warning should only appear when the user opens the file twice in the same session. In addition, "some of your progress" is misleading; because this warning appears on initial load, we can assume that the user hasn't made any edits yet.

[github-actions]

👈 Launch a Binder on branch JasonWeill/jupyter-collaboration/rewrite-warning-message

[ellisonbg]

Thanks @JasonWeill I think this is an improvement. It may help to go a bit further and clarify that the issue is more specifically related to opening the same file in two different formats (notebook and text editor). There are cases where it is fine to open the file twice (e.g. in separate browser tabs) as long as both are in the same format (e.g., two notebook views).

[JasonWeill]

@ellisonbg Updated the message to clarify that this refers to two separate views.

[davidbrochart]

Thanks.