Show new "Your work" sidebar on Projects dashboard

What does this MR do and why?

This is the first step towards Your work sidebar (#384342 - closed). It introduces the your_work_sidebar feature flag, that when enabled adds a "Your work" sidebar to certain pages. In this first MR, it only adds one menu item to keep the MR small and easy to review.

Screenshots or screen recordings

How to set up and validate locally

• Enable the your_work_sidebar feature flag.
• While logged in, go to http://gdk.test:3000/dashboard/projects. This page should show the sidebar now and the Projects item should show as active.
• Click on New project and see the sidebar there, too. Try different screen widths as there is now less space in the main content area for the panels on that page.
• Note that both pages now show breadcrumbs. This was necessary since otherwise there wouldn't be "open sidebar" button on mobile. This brings us also closer to a consistent page layout, always showing breadcrumbs.
• Also note that all other pages that use the dashboard layout now use the sidebar. Example. They will get their own menu items in follow-up MRs.
• Also do a quick sanity check with the feature flag disabled. Nothing should change! No sidebar, no breadcrumbs.

If you wanna get the full picture and a preview of the whole thing, checkout !104276 (closed). That was the "got too huge" MR that I'm splitting up.

MR acceptance checklist

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