Titles of posts aren't respected

When you start a post with a Markdown heading, you usually mean that to be the post's title. But the web pages generated by ssb-viewer always use the (non-human-readable) message ID as the title. This in turn gets scraped by traditional web-based sharing platforms, and makes SSB look like a place for weirdos who actually enjoy staring at encoded SHA256 hashes.

The viewer should parse the markdown and extract the top heading, if any, of the linked post to use as the title.