Use `by @authorname` in each PR note to include a list of images in GitHub releases

[choldgraf]

Context

GitHub releases will put a list of headshots for all contributors to a release at the bottom of the release notes. However, it detects this group by looking for the words by @HANDLE. This theme currently doesn't generate this pattern, and so GitHub misses them.

One downside to this is that the @HANDLE pattern only auto-links on GitHub itself, not in documentation sites etc. Moreover, using [@HANDLE](https://github.com/HANDLE) doesn't work properly either.