Add an Edit Network screen via an action on site-info.php

Overrides site-info.php in the same way that we've overridden sites.php and site-new.php for other Network specific screens
Remove unnecessary check with DOCUMENT_URI

Our forced replacements for new site and the networks screens are loaded via a specific action hook that should not fire unless the appropriate page has been requested. Because of this, we don't have to duplicate efforts by checking the DOCUMENT_URI.