Send as webhook.me.uk, add favicon, share image and SEO tags

Mail now comes from no-reply@webhook.me.uk (DKIM, SPF and DMARC are
set up for it) and the Message-ID uses that domain instead of the
container hostname. The landing page gets a favicon, an Open Graph
image, canonical/OG/Twitter/JSON-LD tags, robots.txt that keeps
crawlers off email URLs, and a sitemap. Footer links to dixon.cx.

static/favicon.svg

@@ -0,0 +1,7 @@
+<svg xmlns="http://www.w3.org/2000/svg" viewBox="0 0 64 64">
+  <rect width="64" height="64" rx="14" fill="#0047ab"/>
+  <rect x="10" y="18" width="38" height="28" rx="4" fill="#fff"/>
+  <path d="M12 21l17 13 17-13" fill="none" stroke="#0047ab" stroke-width="4" stroke-linecap="round" stroke-linejoin="round"/>
+  <circle cx="47" cy="45" r="12" fill="#0a0a0a" stroke="#0047ab" stroke-width="3"/>
+  <path d="M41 45h11m-4-4.5 4.5 4.5-4.5 4.5" fill="none" stroke="#fff" stroke-width="3" stroke-linecap="round" stroke-linejoin="round"/>
+</svg>