Store hash of Markdown in content

To be able to offer programmatic edits to the Markdown source (such as for #15), we need to know that the source hasn't changed since the HTML was generated.

We should generate a hash of the Markdown source and store it in the `post_content`. This will allow us to test whether the hash matches, and hence whether the Markdown source is authoritative. We can then offer editing with confidence, or warn the user if it has been edited elsewhere.