CI: gulp task in support of refcache cleanup & some refcache updates

[chalin]

• Contributes to:
  • CI: external-link check follow up and link cleanup #2429
  • CI: refcache update policy and cron-job proposal #2554
• Adds a Gulp task to prune the n oldest entries from the refcache.json file.
• Refreshes about 150 entries from the refcache.json file. Note that deleted entries are external links that are no longer used.
• The idea will be to eventually setup a cron job to refresh the refcache.