So I really like this pull request template I made. You can read about it here. This chrome extension pops up a little icon in your url bar when you're on a pull request. If you click it, it'll insert it into the pull request body.
The benefit of this over a bookmarklet? It will dynamically fetch the content of your PR template from the configured location, without having issues with cross-origin policy.
Thanks to Rémi (in the comments of blog post above) for the idea!
WARNING: This extension is a big hack that was finished over an evening. Don't judge me based on this code.
https://chrome.google.com/webstore/detail/purty/efclbmjddhkddbchknpahlddnpnadjlm