SES-1686 : Improved external resource link appearance.

* External links hidden by default.
* Hovering over resource explore link causes external link to appear.
* Resources without a repository-defined namespace also have external
  links now.
Fix (hopefully) for build break in commit 6a6dc0e4
Merged in dvisser/sesame (pull request #61)

Fix (hopefully) for build break in commit 6a6dc0e4
Merged in dvisser/sesame/ses-1686 (pull request #58)

SES-1686 : Improved external resource link appearance.