Added feature flag and scaffolding for the paneled design

What does this MR do and why?

This MR adds the feature flag for paneled redesign and does minimal scaffolding for the AI panel and drawer -> panel. The code is not final and should be used as a starting point for redesign workstreams.

Screenshots or screen recordings

Without FF	With FF

How to set up and validate locally

1. In rails console enable the feature flag

   Feature.enable(:paneled_view)

2. Check a few pages (preferable one of them issues list with opening the drawer)
3. Disable the FF and make sure these pages look like current master

MR acceptance checklist

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