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 jupyterlab/jupyterlab#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: jupyterlab/jupyterlab#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.

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