_content: add (silent) Mastodon verification link to template

This is the least-complication by-the-book way of making our
account verify to Mastodon (Hachyderm.io).

Change-Id: I863dd4e327b50a7604de288349836216ac0ae7bc
Reviewed-on: https://go-review.googlesource.com/c/website/+/552815
Reviewed-by: Joedian Reid <joedian@google.com>
TryBot-Result: Gopher Robot <gobot@golang.org>
Run-TryBot: David Chase <drchase@google.com>

_content/site.tmpl

@@ -18,6 +18,7 @@
 <link rel="icon" href="/images/favicon-gopher.png" sizes="any">
 <link rel="apple-touch-icon" href="/images/favicon-gopher-plain.png"/>
 <link rel="icon" href="/images/favicon-gopher.svg" type="image/svg+xml">
+<link rel="me" href="https://hachyderm.io/@golang">
 {{if strings.HasPrefix .URL "/blog/"}}
 <link rel="alternate" title="The Go Blog" type="application/atom+xml" href="/blog/feed.atom">
 {{end}}