Revised hack of static NexSON download links.

This is in the tree viewer (Bibliographic References page). Ideally, we
would add the phylesystem-api domain and study fetch URL to its
private/config file, then read this from the default view_dict, to stay
in sync with configuration changes (adding a ticket #360).
Merge pull request #361 from OpenTreeOfLife/master

Revised hack of static NexSON download links.