Add one-time setup links as an alternative to the QR code

A config can now be handed over as a one-time link, valid for 1 h, 24 h or
7 days and protected by a PIN by default. Keys are made only when the link
is opened; the link works once and is revoked after 5 wrong PINs. Issuing a
new config offers the same choice, and the current config keeps working
until the link is used.

Remove the option to paste a client's public key, in the web UI, the API
and the iOS app.

stats.go

@@ -198,7 +198,9 @@ func (s *Stats) sample() {
 	idByKey := map[string]string{}
 	exists := map[string]bool{}
 	for _, p := range cfg.Peers {
-		idByKey[p.PublicKey] = p.ID
+		if p.hasKey() {
+			idByKey[p.PublicKey] = p.ID
+		}
 		exists[p.ID] = true
 	}
 	now := time.Now()