Link to the GitHub edit page directly where possible

[ScalaWilliam]

So that editing contribution becomes an easy one/two-click thing rather than looking for the repository and finding the right file.

Kind of like here on the right side of the page:
http://www.scala-lang.org/blog/2018/03/19/accessible-scala.html

[swannodette]

This a nice idea but somebody is going to have to do that work.