Auto-link "GH-<num>" in PR titles, similar to "#<num>" #3262

Right now, if you place #SOME_NUMBER in a PR title, you'll get a nice link to the issue that is referenced. This is a Refined Github feature (I think?).

According to the documentation, there are a few other ways of linking issues, and I personally like to link them as "GH-SOME_NUMBER" (they work really well as branch names). It'd be nice if Refined Github also recognized this shorthand when it sees it in a PR title.