Non-local resources should not have Download links

[markw]

At present, a resource which is just a URL link to an external resource has a 'Download' button on the resource page. This gives the misleading impression that the resource is stored locally.

This is related to another small UI issue: I think the URL of a resource should be much more prominent, not buried in the 'Additional Information' table.

Suggested fix:

• Put the URL prominently at the top of the resource page (above the preview) (and make it link to the resource)
• Disable the Download button unless the resource is stored locally.