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.

setup.html

@@ -0,0 +1,18 @@
+<!doctype html>
+<html lang="en">
+<head>
+<meta charset="utf-8">
+<meta name="viewport" content="width=device-width, initial-scale=1">
+<meta name="referrer" content="no-referrer">
+<meta name="robots" content="noindex">
+<title>Set up your VPN · GHOSTWIRE</title>
+<link rel="icon" href="/favicon.svg" type="image/svg+xml">
+<link rel="apple-touch-icon" href="/apple-touch-icon.png">
+<link rel="stylesheet" href="/app.css">
+<script src="/setup.js" defer></script>
+</head>
+<body>
+<div id="app" class="setuppage"><p class="loading-page">Loading…</p></div>
+<noscript><p class="loading-page">This page needs JavaScript.</p></noscript>
+</body>
+</html>