controlclient,health,ipnlocal,tailcfg: add DisplayMessage support

Updates tailscale/corp#27759

Signed-off-by: James Sanderson <jsanderson@tailscale.com>

ipn/ipnlocal/local.go

@@ -5828,7 +5828,14 @@ func (b *LocalBackend) setNetMapLocked(nm *netmap.NetworkMap) {
 	b.pauseOrResumeControlClientLocked()
 
 	if nm != nil {
-		b.health.SetControlHealth(nm.DisplayMessages)
+		messages := make(map[tailcfg.DisplayMessageID]tailcfg.DisplayMessage)
+		for id, msg := range nm.DisplayMessages {
+			if msg.PrimaryAction != nil && !b.validPopBrowserURL(msg.PrimaryAction.URL) {
+				msg.PrimaryAction = nil
+			}
+			messages[id] = msg
+		}
+		b.health.SetControlHealth(messages)
 	} else {
 		b.health.SetControlHealth(nil)
 	}