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>
