Fix click behavior of user dropdown menu item with navigation toggle

What does this MR do and why?

This fixes (removes) the hover and active styles of the dropdown menu item that contains our "switch new navigation on/off" toggle. Before this change, the hover effect gave the impression that the full width of that menu item was actionable, but it is only the toggle itself that switches between old and new navigation.

Also it stops event propagation when the menu item is clicked (but not the toggle). This way the dropdown doesn't close when clicked on the menu item.

Screenshots or screen recordings

It's not noticable in the screen recording, but clicking the menu item but not the toggle doesn't close the dropdown anymore. This has the nice effect that you can see the toggle sliding to the other side before the page reload kicks in.

How to set up and validate locally

Numbered steps to set up and validate the change are strongly suggested.

MR acceptance checklist

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