Global setting for disabling "Edit button"

People are having issues figuring out how to remove the "Edit on Github" button, and I would agree that it's cumbersome.

I don't think it's a good idea to remove it, but I'm sure people have their reasons.

The most straight-forward solution would probably be to respect the `html_show_sourcelink` setting.

See also: https://github.com/sphinx-doc/sphinx/issues/2386