Use GithubRepo case class

[ashawley]

Was reading the source code for Scaladex, and noticed this class type:

case class GithubRepo(organization: String, repository: String)

https://github.com/scalacenter/scaladex/blob/master/model/src/main/scala/ch.epfl.scala.index.model/misc/GithubRepo.scala

A similar implementation exists already in the app of github-api-scala, at

src/main/scala/codecheck/github/app/CommandSetting.scala

Perhaps, use it at other layers of the code.