Add scheme, host when opening relative URLs.

Fixes #2710 
I'm not sure if this would fix properly a link like `/?p=123`, since it would turn it into `http://example.com/`

`+[NSURL URLWithString:relativeToURL:]` feels much safer here, as in `linkURL = [NSURL URLWithString:linkURL.absoluteString relativeToURL:postURL]`