Add extra URLs for a PURL in the UI (and possibly API)

[pombredanne]

Either we compute the repo home homepage or we fetch it from PurlDB, but with a PURL like pkg:npm/[email protected] I would like a link to https://www.npmjs.com/package/log4js/v/6.4.0 and other PURL-related links. Today we just display the PURL which does not help get more details from a Package