SVG rendering cleanup for novice/git/img/github-collaboration.svg

Figuring out why [`<image>` values weren't rendering][1]
(swcarpentry/bc#763) took a bit of digging, and I cleaned up a few
other bits while I was at it.  Details in the commit messages.

[1]: https://developer.mozilla.org/en-US/docs/Web/SVG/SVG_as_an_Image#Restrictions