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.

config.go

@@ -118,8 +118,15 @@ type Peer struct {
 	Keepalive    *int       `json:"keepalive,omitempty"`  // nil = server default
 	Created      time.Time  `json:"created"`
 	ConfigIssued *time.Time `json:"configIssued,omitempty"`
+	// Setup is a pending one-time setup link. A peer created with a link has
+	// no public key until the link is opened.
+	Setup *SetupLink `json:"setup,omitempty"`
 }
 
+// hasKey reports whether the peer has a public key, i.e. it can be in the
+// kernel. A peer waiting for its setup link has none yet.
+func (p *Peer) hasKey() bool { return p.PublicKey != "" }
+
 type LogConfig struct {
 	Level     string `json:"level"` // debug | info | warn | error
 	MaxSizeMB int    `json:"maxSizeMB"`
@@ -346,7 +353,7 @@ func (c *Config) validate() error {
 			return fmt.Errorf("address %s is used twice", ip)
 		}
 		ips[ip] = true
-		if keys[p.PublicKey] {
+		if p.hasKey() && keys[p.PublicKey] {
 			return fmt.Errorf("peer %q: public key is used by another peer", p.Name)
 		}
 		keys[p.PublicKey] = true