client/web: button, link, and other small UI updates

Makes the following changes:
* Use “link” class in various spots
* Remove button appearance on Exit Node dropdown in readonly mode
* Update `-stone-` colors to `-gray-` (couple spots missed by
  original color config commit)
* Pull full ui/button component from admin panel, and update
  buttons throughout UI to use this component
* Remove various buttons in readonly view to match mocks
* Add route (and “pending approval”) highlights to Subnet router
  settings card
* Delete legacy client button styles from index.css
* Fix overflow of IPv6 address on device details view

Updates #10261

Signed-off-by: Sonia Appasamy <sonia@tailscale.com>

client/web/src/ui/collapsible.tsx

@@ -24,7 +24,7 @@ export default function Collapsible(props: CollapsibleProps) {
         onOpenChange?.(open)
       }}
     >
-      <Primitive.Trigger className="inline-flex items-center text-gray-600 cursor-pointer hover:bg-stone-100 rounded text-sm font-medium pr-3 py-1 transition-colors">
+      <Primitive.Trigger className="inline-flex items-center text-gray-600 cursor-pointer hover:bg-gray-100 rounded text-sm font-medium pr-3 py-1 transition-colors">
         <span className="ml-2 mr-1.5 group-hover:text-gray-500 -rotate-90 state-open:rotate-0">
           <ChevronDown strokeWidth={3} className="stroke-gray-400 w-4" />
         </span>