one snackbar list, css transitions for the drawer

[pfefferle]

Proposed changes:

Two things that came out of looking at our Layout next to boot's shell.

• Toasts showed twice. Boot's root already renders the snackbar notices from core/notices, and our Layout rendered a second list for the same notices. Ours is gone.
• The mobile sidebar drawer and its backdrop were animated with __unstableMotion and __unstableAnimatePresence from @wordpress/components. They are CSS transitions now, with prefers-reduced-motion respected through the base-styles mixin. Same 0.2s fade and slide as before.

Other information:

• Have you written new tests for your changes, if applicable?

Testing instructions:

• Open the Social Web screen, follow or block someone from the followers actions (or trigger any snackbar). One toast, not two.
• Narrow the window below 782px, open the menu with the header button: the drawer slides in and the backdrop fades. Close it via the backdrop or Escape: it slides out. With "reduce motion" enabled in the OS, both switch without animation.