Not possible to preview social media images on forked repos

[mgriffin-scottlogic]

When forking this repo to create blog posts, if an image: is specified in the post metadata, the generated preview page will point to the location on blog.scottlogic.com where the image does not exist yet. This results in social media preview tools like https://www.linkedin.com/post-inspector/ falling back on the first image it can find in the page, which is usually the profile picture, which may be another manifestation of #37.

Note: I think this can be resolved by switching the use of site.canonical.url in _layouts/default.html with site.github.url. I'm not sure if there is much use for site.canonical.url in general but there are a few more uses of it in the codebase.