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.

.dockerignore

@@ -2,3 +2,4 @@
 !go.mod
 !*.go
 !docs.html
+!static