For specific repos, redirect edit button links to target master

[kilfoyle]

Thanks to Greg, we've recently introduced the edit button in the Cloud docs. The Cloud docs are a bit of a special case both because the repos are private and because we don't build the master branches. Since all doc updates should go into the master branch first, this means that all PRs opened via the edit button need to be rebased. This is proving to be pretty inefficient. It would be a wonderful, time and effort-saving feature if, for select repos like Cloud, the edit button link could redirect to always target the relevant source file in master.