The major change is moving the decision of whether to use oEmbed
further up the call-stack. This reverts the _download_url method to
being a "dumb" functionwhich takes a single URL and downloads it
(as it was before #7920).
This also makes more minor refactorings:
* Renames internal variables for clarity.
* Factors out shared code between the HTML and rich oEmbed
previews.
* Fixes tests to preview an oEmbed image.
Point to the book where possible, and use hyperlinks to github to refer to files not included in the book.
Co-authored-by: Andrew Morgan <1342360+anoadragon453@users.noreply.github.com>
This is essentially an implementation of the proposal made at https://hackmd.io/@richvdh/BJYXQMQHO, though the details have ended up looking slightly different.
