Add a basic flyout menu

What does this MR do and why?

This adds a basic flyout menu to the POC. Initially the submenu items are rendered on the server as they did before. When flyout conditions are met the menu rendered on the BE side is repositioned to the flyout menu. I've used popper.js for positioning as it makes things much simpler. We could also render the app/views/shared/nav/_sidebar_submenu.html.haml in Vue. But in this iteration I have not figured correctly how to pass submenu items to the frontend. The only difference with the current approach that popper. is used for positioning thus logic is much simpler.

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Related to #383098 (closed)