It would be nice if you could just drag a link to your toolbar, but GitHub sanitizes javascript:
links in Markdown (which is a good thing, really). Therefore you'll have to do this manually. Create a bookmark with the following URL content:
javascript:(function()%7Bvar%20s%3Ddocument.createElement('script')%3Bs.src%3D'https%3A%2F%2Fgist.githack.com%2Flackac%2F03c6d1921bde0b97e050%2Fraw%2Fcollapsible_jira_board.js'%3Bdocument.getElementsByTagName('script')%5B0%5D.parentNode.appendChild(s)%7D)()