Add links to the developers’ GitHub profiles

This pull request adds a “Who made this?” item to the Sage Cell Server page which provides links to the GitHub profiles of @jasongrout, @ihanson, @kramer314, and @stevenJohnson, as well as to the [project repository](https://github.com/sagemath/sagecell) on GitHub. (One impetus for this change is that I would like to link to http://aleph.sagemath.org in my grad school applications, and it would be useful to be able to prove that I helped create it as well as to link to my GitHub account.)

Are there any other requests for links to include in this box?