Change allow_hidden config default for easier hidden files access

[tmchartrand]

Problem

It's still needlessly involved as a user to enable viewing hidden files from jupyter/jupyterlab, leading to issues like https://github.com/jupyterlab/jupyterlab/issues/13108. Since there is a UI setting to enable the viewing, it seems most users don't expect to need to change the server config also. If I understand correctly, it seems the original intent when adding the UI setting was to change the default, but there were technical issues in doing so without overwriting user sessions: https://github.com/jupyterlab/jupyterlab/pull/10769.

Proposed Solution

If it is now possible to do so easily, change the default allow_hidden to True, along with any other settings that may be needed to enable the user to activate hidden file viewing through the UI.