cli: web advertise exit node button

A couple of gnarly assumptions in this code, as always with the async
message thing.

UI button is based on the DNS settings in the admin panel.

Co-authored-by: Maisem Ali <maisem@tailscale.com>
Signed-off-by: David Crawshaw <crawshaw@tailscale.com>
This commit is contained in:
Internal Server Error - Gitea: Git with a cup of tea
500 Internal Server Error

Gitea Version: 1.28.0+dev-477-g8b6ad49a5f